fix(python): avoid async work in sync reprs (#3620)

## Summary

- keep the existing synchronous `connect()` path unchanged
- make `LanceDBConnection.__repr__` and `LanceTable.__repr__`
side-effect-free
- add a regression test that verifies sync reprs do not call the Python
background loop

## Root cause

The freeze is caused by debugger rendering, not by `connect()` itself:

1. debugpy stops at a breakpoint and suspends all Python threads.
2. The debugger renders the new `db_connection` local by calling
`repr()`.
3. `LanceDBConnection.__repr__` reads `read_consistency_interval`.
4. That property calls `LOOP.run(...).result()`.
5. The `LanceDBBackgroundEventLoop` thread is suspended by the debugger,
so `repr()` waits for a thread that cannot run.

This explains why the symptom appears immediately after `connect()`: it
is the first point where a connection object exists in locals and is
automatically rendered. `LanceTable.__repr__` had the same problem
because it also read the connection's consistency interval.

This follows the same principle as #3411: `__repr__` must not trigger
async work or I/O that a debugger assumes is lightweight.

## Evidence

I reproduced the behavior with the real LanceDB classes and debugpy
1.8.21 using a DAP client:

- latest `main` (`ff6ff099`): the debugger reported `allThreadsStopped:
true`, and evaluating `repr(db_connection)` timed out
- this branch (`5755a5ba`): the same evaluation returned
`LanceDBConnection(uri='/tmp/lancedb-debug-repro')` immediately
- setting `PYDEVD_UNBLOCK_THREADS_TIMEOUT=0` also allowed the original
repr path to complete, independently confirming that it was waiting on a
suspended thread

The regression test creates a connection and table, replaces `LOOP.run`
with a function that fails, and verifies that both reprs still work.

## Validation

- `maturin develop --manifest-path python/Cargo.toml`
- `python -m pytest
python/python/tests/test_db.py::test_sync_repr_does_not_use_background_loop
python/python/tests/test_table.py::test_consistency -q` (`4 passed`)
- `ruff check .`
- `ruff format --check python/python/lancedb/db.py
python/python/lancedb/table.py python/python/tests/test_db.py
python/python/tests/test_table.py`
- `git diff --check`

Refs #3611.

python/python/lancedb/db.py

@@ -688,11 +688,7 @@ class LanceDBConnection(DBConnection):
         return cls(None, _inner=inner)
 
     def __repr__(self) -> str:
-        val = f"{self.__class__.__name__}(uri={self._conn.uri!r}"
-        if self.read_consistency_interval is not None:
-            val += f", read_consistency_interval={repr(self.read_consistency_interval)}"
-        val += ")"
-        return val
+        return f"{self.__class__.__name__}(uri={self._conn.uri!r})"
 
     @override
     def serialize(self) -> str:

python/python/lancedb/table.py

@@ -2468,13 +2468,7 @@ class LanceTable(Table):
         return LOOP.run(self._table.count_rows(filter))
 
     def __repr__(self) -> str:
-        val = f"{self.__class__.__name__}(name={self.name!r}"
-        if self._conn.read_consistency_interval is not None:
-            val += ", read_consistency_interval={!r}".format(
-                self._conn.read_consistency_interval
-            )
-        val += f", _conn={self._conn!r})"
-        return val
+        return f"{self.__class__.__name__}(name={self.name!r}, _conn={self._conn!r})"
 
     def __str__(self) -> str:
         return self.__repr__()

python/python/tests/test_db.py

@@ -62,6 +62,21 @@ def test_basic(tmp_path):
     assert db.open_table("test").name == db["test"].name
 
 
+def test_sync_repr_does_not_use_background_loop(tmp_path, monkeypatch):
+    from lancedb.background_loop import LOOP
+
+    db = lancedb.connect(tmp_path)
+    table = db.create_table("test", data=[{"id": 1}])
+
+    def fail_run(*args, **kwargs):
+        raise AssertionError("repr should not use the Python background loop")
+
+    monkeypatch.setattr(LOOP, "run", fail_run)
+
+    assert repr(db) == f"LanceDBConnection(uri={str(tmp_path)!r})"
+    assert repr(table) == f"LanceTable(name='test', _conn={db!r})"
+
+
 def test_ingest_pd(tmp_path):
     db = lancedb.connect(tmp_path)
 

python/python/tests/test_table.py

@@ -3087,9 +3087,6 @@ def test_consistency(tmp_path, consistency_interval):
 
     db2 = lancedb.connect(tmp_path, read_consistency_interval=consistency_interval)
     table2 = db2.open_table("my_table")
-    if consistency_interval is not None:
-        assert "read_consistency_interval=datetime.timedelta(" in repr(db2)
-        assert "read_consistency_interval=datetime.timedelta(" in repr(table2)
     assert table2.version == table.version
 
     table.add([{"id": 1}])