[fix] KeyError crash when completing right after USE

[scottnemes]

Description

Issue

Running USE immediately points the live completer at the new schema so unqualified completions reflect the switch, but that schema's tables are still loading on a background thread. If you triggered a completion in that window, mycli crashed:

File ".../mycli/sqlcompleter.py", line 1757, in populate_scoped_cols
    for table in meta["tables"][self.dbname]:
                 ~~~~~~~~~~~~~~^^^^^^^^^^^^^
KeyError: 'new_db'

To reproduce (difficult unless you have a large database and/or slow computer):

USE some_db;
SELECT 

Fix

populate_scoped_cols now reads the per-schema metadata with meta["tables"].get(self.dbname, {}), snapshotting it once. During the load window it suggests * instead of raising an error, and shows real columns once the refresh completes. Grabbing the reference once also avoids a concurrent refresh swapping the dict out mid-iteration.

Added regression tests in test_completer_use_switch.py covering the unloaded-schema window, the post-load result, and concurrent USE switching.

Fixes #1933

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .