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/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}])