Add test coverage for mycli/key_bindings.py

[rolandwalker]

Description

Add extensive test coverage for mycli/key_bindings.py.

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]

Codex Review

1. Low: filter-patching helper is brittle to prompt_toolkit internals
   test_key_bindings.py:154 patches emacs_mode.func.__globals__, completion_is_selected.func.__globals__, and control_is_searchable.func.__globals__. This relies on internal structure (.func and module globals) and can break on upstream refactors without behavior changes in mycli.
   Action: prefer exercising filters through an app/session fixture (or patching via stable public hooks) instead of mutating filter function globals.

2. Low: missing edge-case coverage for combined key behaviors
   test_key_bindings.py:292 and test_key_bindings.py:362 only test single behavior entries (advance, cancel, etc.). mycli/key_bindings.py supports lists, and mixed configs (for example including toolkit_default plus another behavior) can trigger multi-step paths.
   Action: add parametrized cases for mixed behavior lists and assert the exact precedence/side effects.

No correctness or security bugs were found in the PR’s production code path (this PR only adds tests).
I could not execute the new tests locally because uv is unavailable in this environment (uv: command not found).