Break sqlcompleter.py find_matches() into units, and add test coverage.

[rolandwalker]

Description

This also changes find_matches() into an instance method, but we could consider changing find_matches() and many others into static methods.

Motivation: smaller units make the code more testable and more amenable to agentic coding.

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 .

[github-actions]

1. Potential compatibility regression: find_matches is no longer callable as a class/static method
   In this PR, find_matches changed from @staticmethod to an instance method (self first arg) at mycli/sqlcompleter.py:1304.
   Any external usage like SQLCompleter.find_matches("sel", collection) now raises TypeError instead of working as before.
   Action: either restore static compatibility (keep find_matches static and refactor helpers accordingly), or explicitly document this as a breaking API change and add a regression test asserting intended calling style.

2. Missing test for the above API behavior
   New tests cover internals heavily, but none verify whether class-level invocation remains supported after refactor at test/pytests/test_sqlcompleter_find_matches.py.
   Action: add one test for SQLCompleter.find_matches(...) behavior (supported or intentionally unsupported) to lock in expected API behavior.

I didn’t find a new security issue in this diff.
I could not run the tests in this environment because uv is unavailable.