fix(python): implement remote namespace and table existence checks (#4725)

Remote Python connections inherited the unsupported-operation stubs for
`namespace_exists` and `table_exists`, so both raised
`NotImplementedError` even when namespace and table operations worked.

Add the missing remote overrides, async methods, and PyO3 bindings using
the existing namespace describe and table open paths. Only an explicit
not-found result returns `False`; permission, transport, and server
errors propagate. This preserves authentication, dynamic headers,
retries, and nested namespace handling, and rechecks cached tables
before reporting that they exist.

The async namespace helper also recognizes native `NamespaceNotFound`
errors so missing local namespaces return `False`. The table and
namespace guides document Python remote support and distinguish it from
the unsupported `exists` routes used by direct REST namespace clients.

Validation:
- All 40 focused regression cases pass, covering successful and missing
lookups, local and remote namespaces, dynamic headers, permission/server
errors, invalid table identifiers, and cached tables that disappear.
- The original inherited stubs reproduce the reported
`NotImplementedError`; the local namespace regression reproduced the
native missing-namespace `RuntimeError` before its fix.
- Full remote connection suite: 255 passed. Local namespace tests: 14
passed.
- `cargo clippy --quiet -p lancedb-python --features remote --lib`,
`cargo fmt --all`, Python Ruff lint, and Python formatting checks pass.
- Root `ruff check .` reports 48 existing errors in documentation
examples under `docs/web-tests`; the Python subtree passes and those
unrelated files are unchanged.

Fixes #4722

<!-- lance-gatekeeper-fix:v1 agent=4d369a0681756fdb5a8b35853d7f5607
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

docs/web/management/namespace.mdx

@@ -243,8 +243,11 @@ let db = lancedb::connect_namespace("rest", properties)
 ```
 </CodeGroup>
 [LanceDB Enterprise](/enterprise) operates a REST namespace server on top of the Lance format, so any REST client that can speak the REST namespace API
-contract can be used to interact with it. It does not serve the contract's `exists` routes, so checks such as `namespace_exists`
-fail against it. For authentication examples in LanceDB Enterprise, visit
+contract can be used to interact with it. It does not serve the contract's `exists` routes,
+so existence checks made through a direct REST namespace client fail against it.
+Python remote connections created with `lancedb.connect("db://...")` or
+`lancedb.connect_async("db://...")` support `namespace_exists` and `table_exists`
+through the existing describe routes instead. For authentication examples in LanceDB Enterprise, visit
 the [Namespaces in SDKs](#namespaces-in-lancedb-enterprise) page. 
 
 ## Best practices {#best-practices}

docs/web/management/table.mdx

@@ -214,14 +214,21 @@ await db.tableNames();
 Each accepts `namespace_path` to act inside a [namespace](/management/namespace)
 rather than at the database root.
 
-To find out whether a table exists, open it and handle the error, or look for
-its name in a listing. A missing table raises `ValueError` on a local directory
-or a deployment, and `lance_namespace.TableNotFoundError` on a connection from
-`connect_namespace`. A listing returns one page of names at a time, so check
-every page. Python's `table_exists` is no substitute: a local directory or a
-deployment raises `NotImplementedError` for it, and on a connection from
-`connect_namespace("dir", ...)` or `connect_namespace("rest", ...)` it fails with
-an `AttributeError`. TypeScript has no `tableExists`.
+For Python remote connections created with `lancedb.connect("db://...")`, use
+`db.table_exists(["documents"])` to check whether a table exists. For an async
+remote connection, use `await db.table_exists(["documents"])`. Include namespace
+segments before the table name, for example `["analytics", "documents"]`. These
+checks return `False` when the table is missing and propagate other errors,
+including permission errors.
+
+Local synchronous directory connections still raise `NotImplementedError` for
+`table_exists`. On a connection from `connect_namespace("dir", ...)` or
+`connect_namespace("rest", ...)`, it fails with an `AttributeError`. For these
+connections, open the table and handle the error, or look for its name in a
+listing. A missing table raises `ValueError` on a local directory and
+`lance_namespace.TableNotFoundError` on a connection from `connect_namespace`.
+A listing returns one page of names at a time, so check every page. TypeScript
+has no `tableExists`.
 
 ## Rename and clone tables {#rename-clone}