Improve composition check for keybindings

[colin-grant-work]

What it does

Fixes a few issues with IME composition that I've observed typing in Korean (on MacOS). The issues were:

• Problems in transition between syllable blocks. Intermittently(?), typing a multi-block word with a forced break would ignore the first letter of the second block.
• Problems transitioning out of composition through other keypresses: e.g. down arrow would insert current block at current position, and also insert it in the current cursor index in the next line, and then move to the second line after the initial input.

How to test

1. Using a Korean keyboard, type a variety of syllable blocks. (I used two-set Korean, so these sequences should give these outputs: akfrdms 맑은 akfrms 말근 aksen 만두 dhkdwk 왕자)
2. Using a Korean keyboard, in a file with some text (I used packages/ai-mcp-ui/src/browser/mcp-command-contribution.ts), type one of the syllable blocks above on one line, then, before exiting composition (i.e. without typing space or enter), hit down arrow. The text you input should appear on the line on which you input it, and not on the line below. i.e. you should not end up with this:

3. Try some other IME systems and make sure nothing is broken. I tried locally with Chinese (Zhuyin) input and Japanese.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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