Optionally defer completions until N characters have been typed

[rolandwalker]

Description

A strong effort is taken for efficiency on reading the trailing characters, since this prompt_toolkit Filter will run on every keystroke. Though the cost of running the Pygments lexer on every keystroke surely dwarfs this.

If fewer than N characters have been typed, the suggestions can still be summoned by control-space (does not advance into the candidates) or tab (does immediately advance into the candidates).

The motivation is to both reduce distractions and to reduce lag when typing.

There is another way to do this by passing thresholds into mycli/sqlcompleter.py, but it doesn't preserve the ability to summon completions when below the trigger threshold.

Tests are a challenge again because we would need a custom myclirc.

I could almost be convinced to change the default trigger threshold to 3 for all users. It's better! But changing defaults is a bad practice in principle.

xref #1548

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 .

[scottnemes]

Good option for anyone who wants to change it anyway! Tested out various values of the new option and works as expected.

[github-actions]

Findings

1. Potential cross-instance behavior leak (minor correctness).
   MIN_COMPLETION_TRIGGER is a module global that gets overwritten in each MyCli init. If multiple MyCli instances exist in one process (tests, embedded use), the last-initialized instance controls behavior for all. Consider making complete_while_typing_filter an instance-bound Condition or closing over self.min_completion_trigger so each session is isolated.
   File: mycli/main.py

2. Missing tests for the new filter logic and config behavior.
   This PR changes completion behavior based on a config value and special-cases source/\.. There are no tests to cover:

• min_completion_trigger > 1 enabling/disabling auto-complete.
• The source/\. path punctuation rule.
• Regression that completions can still be triggered via tab/ctrl-space below threshold (behavior described in PR, but untested).
  Files: mycli/main.py, test/myclirc

Open questions / assumptions

• Is it acceptable for min_completion_trigger <= 1 to always enable auto-complete (no validation)? If the intended range is >= 1, consider clamping or documenting.
  File: mycli/main.py

If you want, I can suggest a minimal test strategy that avoids full prompt_toolkit integration.