fix(shell): reduce hidden modal input latency

[bugkeep]

Summary

• skip completion startup when the active prompt delegate hides the input buffer
• let hidden-input modals use the idle refresh interval unless a fast-refresh provider explicitly requests faster redraws
• add focused tests for hidden modal completion suppression and refresh interval selection

Tests

• uv run pytest tests\ui_and_conv\test_prompt_tips.py -q -k "hidden_modal_text_change or visible_prompt_text_change or hidden_modal_uses_idle or hidden_modal_keeps_fast or visible_running_prompt_uses_running"
• uv run ruff check src\kimi_cli\ui\shell\prompt.py tests\ui_and_conv\test_prompt_tips.py
• uv run ruff format --check src\kimi_cli\ui\shell\prompt.py tests\ui_and_conv\test_prompt_tips.py
• uv run pyright src\kimi_cli\ui\shell\prompt.py tests\ui_and_conv\test_prompt_tips.py

Improves #2032

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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