Fix IME Enter submission handling

[ArthurXi]

Summary

• Fix IME Enter handling in the Agent chat input.
• Prevent accidental session submission while text composition is active or has just ended.

Why

When using Chinese/Japanese/Korean IMEs, pressing Enter can be used to confirm composition. The chat input previously treated that Enter as a submit action, which could accidentally trigger a session before the user finished typing.

Changes

• Track IME composition state with onCompositionStart and onCompositionEnd.
• Ignore Enter submit while composition is active.
• Add a short guard after composition ends to avoid browser event-order edge cases.

Test Plan

• Existing tests pass (pytest --ignore=agent/tests/e2e_backtest --tb=short -q)
• New tests added (if applicable)
• Tested manually (describe below)

Manual test:

• Verified that pressing Enter while using an IME to confirm text does not submit the Agent chat.
• Verified that normal Enter still submits after composition has completed.

Checklist

• No changes to protected areas (src/agent/, src/session/, src/providers/) without prior discussion
• No hardcoded values (API keys, file paths, magic numbers)
• Code follows CONTRIBUTING.md guidelines
• Documentation updated (if user-facing change)

[warren618]

Merged — thanks @ArthurXi! 🙏

This is a real fix for CJK users: confirming a composition with Enter no longer fires an accidental submit. Verified the composition-state tracking (isComposing / keyCode 229 + the post-composition guard) behaves correctly and normal Enter-to-submit still works; TypeScript typecheck is clean. Appreciate the clear write-up of the browser event-order edge case.