Let completion-key bindings be configurable

[rolandwalker]

Description

Let completion-key bindings be configurable, but set to the current behaviors by default.

There are several behaviors, some of which are mutually exclusive. It would be complex to lay out in the options which are exclusive, so that is just documented in the commentary.

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. Correctness regression: single-value config is parsed as str, causing wrong behavior matching

   • In both handlers, behaviors can be a string (ConfigObj returns str when only one value is set, e.g. tab = advancing_summon), but code uses substring membership checks.
   • Example: 'summon' in 'advancing_summon' is True, so advancing_summon incorrectly executes the summon branch.
   • Affects:
     • mycli/key_bindings.py:128
     • mycli/key_bindings.py:143
     • mycli/key_bindings.py:145
     • mycli/key_bindings.py:174
     • mycli/key_bindings.py:188
     • mycli/key_bindings.py:190
   • Action: normalize behaviors to a list of exact tokens before checks (e.g. if isinstance(behaviors, str): behaviors = [behaviors]), then keep exact membership matching.

2. Missing tests for new configurable behavior paths

   • The PR changes runtime key behavior but adds no behavioral tests, only config template updates.
   • Action: add tests covering at least:
     • single-value config (tab = advancing_summon, tab = prefixing_summon, control_space = advancing_summon)
     • default multi-value config
     • toggle behavior when completion menu is open.

Security review: no new security concerns identified in this diff.