fix(prompt-input): prevent extra line break after send

[heath-s]

Summary

• consume Enter commands that the prompt composer already handled
• prevent Lexical from inserting a trailing line break after message submission
• preserve multiline Enter behavior and add focused regression coverage

Root cause

Lexical's plain-text Enter handler does not honor defaultPrevented, so the composer could submit the prompt while Lexical still inserted a line break. The cleared editor then retained that empty second line and caret position.

Verification

• bun test tests/prompt-lexical-editor.commands.test.ts tests/prompt-input-queue-mode.test.tsx tests/custom-theme.test.ts (41 passed)
• bun run typecheck
• bun run build