Add warnings-count prompt format strings

[rolandwalker]

Description

• \w - number of warnings, or "(none)" if none
• \W - number of warnings, or the empty string

This is an alternative to having SHOW WARNINGS turned on. An example is shown in the prompt, but it is likely to be more useful in the toolbar.

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 risk: \w and \W can disagree in the same prompt

• In mycli/main.py:1550 and mycli/main.py:1556, get_warning_count() is called separately for \w and \W.
• If a prompt contains both tokens, this issues two SHOW COUNT(*) WARNINGS statements. That can produce inconsistent values (and always adds an extra round-trip).
• Action: fetch warning count once per prompt render and reuse it for both replacements.

2. Missing tests for the new feature

• PR adds behavior in mycli/main.py:1550 and helper in mycli/packages/special/utils.py:86, but no tests validate prompt expansion for \w/\W.
• Action: add tests for:
  • warnings present (\w -> count, \W -> count),
  • no warnings (\w -> (none), \W -> empty),
  • both tokens in one prompt (single count lookup / consistent output),
  • disconnected/no-conn fallback behavior.

No direct security issues found in this PR.