Improve provider configuration and chat reliability

[pearlq12345]

Summary

• add provider status, save, and test endpoints for the web settings flow
• improve custom provider and Codex provider handling for model discovery and text fallback
• add provider API client helpers and settings UI for save/test feedback
• surface chat send failures in the chat panel instead of dropping them silently
• cover provider API behavior and text tool fallback paths

Manual validation

• exercised the provider settings flow from the local web UI
• checked provider status loading, save feedback, test feedback, model discovery handling, and invalid key messaging
• verified chat send errors surface in the panel instead of being dropped silently

Verification

• python -m pytest tests/test_web_provider_api.py tests/test_provider_text_tool_fallback.py tests/test_commands.py -q
• cd ui && npm run build

Stack

Depends on #109, #111, and #112.