feat: add asynchronous remote SQL queries (#4070)

## Summary

Add SQL execution to remote LanceDB connections. On the standard
synchronous connection, `execute_query` waits for the initial result
stream and returns its Arrow reader. `execute_query_async` is called
without Python `await` and immediately returns a query handle for status
inspection, streaming, or cancellation. Local databases report that SQL
is not supported.

The transport and query lifecycle live in Rust. Python exposes
native-backed synchronous and asynchronous connection methods and query
wrappers; it does not use PyArrow's Flight client.

## User experience

The standard synchronous connection supports both direct reads and
background query execution:

```python
db = lancedb.connect(
    "db://analytics",
    api_key="ldb_...",
    sql_host_override="grpc+tls://sql.example.com:10026",
)

# Direct execution waits only until the initial result stream is available.
# Later batches continue streaming as the query progresses.
reader = db.execute_query(
    "SELECT * FROM events",
    default_namespace_path=["production"],
)
for batch in reader:
    print(batch.num_rows)

# Background execution returns a query handle immediately. Despite the
# `_async` suffix, no Python `await` is needed on a synchronous connection.
query = db.execute_query_async("SELECT * FROM events")
print(query.id)

description = db.describe_query(query.id)
print(description.status)
print(description.progress)
print(description.expires_at)

# Start reading as soon as the service advertises partial results. The reader
# continues polling and yields newly available record batches until the query
# and all result endpoints are complete.
reader = query.reader()
for batch in reader:
    print(batch.num_rows)

# Or cancel a different still-running query. Its status becomes "cancelling"
# while the server is still working, then "cancelled" once confirmed.
cancelled_query = db.execute_query_async("SELECT * FROM large_events")
cancelled_query.cancel()
```

The less commonly used asynchronous connection exposes the same
operations as coroutines:

```python
async_db = await lancedb.connect_async(
    "db://analytics",
    api_key="ldb_...",
    sql_host_override="grpc+tls://sql.example.com:10026",
)
query = await async_db.execute_query_async("SELECT * FROM events")
async for batch in await query.reader():
    print(batch.num_rows)
```

The UUIDv7 query id is scoped to the connection that submitted it. The
connection retains lightweight shared query state used by
`query.describe()` and `db.describe_query(query.id)`; the id does not
encode SQL or a Flight continuation token and is not a cross-connection
resume token. Abandoned state has bounded retention, and terminal state
remains available briefly.

Unqualified table names use the connected database and the `public`
namespace by default. `default_namespace_path` accepts a list such as
`["production", "events"]`. SQL can still use qualified names to
reference other databases and namespaces available to the deployment.

## Design

- Uses Arrow Flight `PollFlightInfo` for submission and long polling,
`DoGet` for results, and `CancelFlightInfo` for cancellation. Each
`PollInfo.info` is treated as the cumulative set of currently available
endpoints, so advertised tickets are consumed once and batches can be
delivered before execution is complete.
- Serializes result completion and cancellation into one lifecycle. A
server-accepted request reports `cancelling` and wakes blocked
status/result work; a later retry can confirm `cancelled`. Result
retrieval is rejected after cancellation is accepted, while cancellation
after a result was already delivered is a no-op.
- Assigns a time-ordered UUIDv7 connection-scoped query id and retains
only shared evolving lifecycle state, keeping SQL, Flight continuation
tokens, and Arrow result data out of public ids and the registry.
- Leaves admission control to the server while honoring server
expiration and a local fallback retention window for abandoned entries.
- Retains terminal ids for five minutes so they remain available for
connection-level description.
- Keeps one lazily initialized SQL client on each remote database
connection and attaches fresh authentication, routing, namespace, and
request metadata to every operation.
- Applies the configured overall timeout to each execution, description,
reader, and cancellation operation. A result reader carries one absolute
deadline from `reader()` through the end of streaming; connect and read
timeouts continue to bound their individual phases.
- Returns a bounded, backpressured, single-consumer Arrow stream rather
than collecting the full result in memory. Dropping the reader stops
downloading but does not implicitly cancel the server query.
- Preserves typed schemas for empty result sets through the stream
schema.
- Accepts Flight result messages up to 1 GiB so a valid row containing a
large blob, string, or vector is not rejected by tonic's 4 MiB default
receive limit.
- Supports the Python client first while keeping the authoritative
implementation in the Rust core.

docs/src/python/python.md

@@ -28,6 +28,59 @@ is also an [asynchronous API client](#connections-asynchronous).
 
 ::: lancedb.Session
 
+## Remote SQL
+
+Submit SQL against a remote LanceDB database through the connection.
+The connected database and `default_namespace_path=["public"]` are used for
+unqualified tables. Fully qualified references can still query other databases
+and namespaces available to the same deployment. `execute_query` returns a
+reader as soon as its initial result stream is available. `execute_query_async`
+returns a query handle immediately; use it to inspect progress, open a reader,
+or cancel the query. The SQL client is initialized by the first query and
+retained for the lifetime of the remote connection. Query ids are random,
+connection-scoped references rather than encoded SQL or durable resume tokens:
+
+```python
+import lancedb
+
+db = lancedb.connect(
+    "db://analytics",
+    api_key="ldb_...",
+    host_override="https://api.example.com",
+    sql_host_override="grpc+tls://sql.example.com:10026",
+)
+reader = db.execute_query(
+    """
+    SELECT events.id, accounts.name
+    FROM analytics.public.events AS events
+    JOIN users.public.accounts AS accounts ON events.user_id = accounts.id
+    """,
+    default_namespace_path=["public"],
+)
+for batch in reader:
+    print(batch.num_rows)
+
+query = db.execute_query_async("SELECT * FROM events")
+print(query.id)
+print(query.describe().status)
+for batch in query.reader():
+    print(batch.num_rows)
+
+# The async connection exposes the same lifecycle without blocking:
+# async_db = await lancedb.connect_async(
+#     "db://analytics",
+#     api_key="ldb_...",
+#     host_override="https://api.example.com",
+#     sql_host_override="grpc+tls://sql.example.com:10026",
+# )
+# reader = await async_db.execute_query("SELECT * FROM events")
+# query = await async_db.execute_query_async("SELECT * FROM events")
+# description = await async_db.describe_query(query.id)
+# async for batch in await query.reader():
+#     print(batch.num_rows)
+# await query.cancel()
+```
+
 ## Namespaces (Synchronous)
 
 A namespace-backed connection resolves tables through a
@@ -102,6 +155,12 @@ listing a storage directory.
 
 ::: lancedb.job.AsyncJob
 
+::: lancedb.sql.Query
+
+::: lancedb.sql.AsyncQuery
+
+::: lancedb.sql.QueryDescription
+
 ## Materialized Views (Synchronous)
 
 ::: lancedb.materialized_view.MaterializedView