This pull request preserves modifier keys on computer mouse actions instead of silently dropping them before driver execution.

What changed

• add optional keyword-only keys support to Computer and AsyncComputer mouse-action methods
• thread action.keys through computer action dispatch for click, double_click, move, scroll, and drag
• raise a clear ModelBehaviorError when a modifier-bearing action reaches a legacy driver that has not implemented keys
• update the Playwright computer example to hold and release modifiers around mouse actions
• add regression coverage and align test doubles with the updated interface
• refresh the English computer-tool docs snippet to show the modifier-capable interface

Verification

1. uv sync --all-extras --all-packages --group dev
2. uv run ruff format
3. uv run ruff check --fix
4. uv run ruff check
5. targeted mypy on changed files
6. targeted pyright on changed files
7. targeted pytest on impacted suites: 374 passed

Notes

Full repo-wide Windows verification still shows unrelated baseline issues in MCP and SQLite/tempfile cleanup paths; those are outside this patch.