[Bug] Chat composer: ArrowUp/ArrowDown recall message history mid-text, blocking multi-line caret navigation

### Description

`ChatComposerInput` (the contentEditable chat composer) uses ArrowUp/ArrowDown to navigate message history regardless of the caret's position, calling `preventDefault()` — so in a multi-line message the caret can't move between lines. ArrowUp replaces the current draft with the previous message; ArrowDown steps through recalled history the same way.

**Expected:** when the caret is mid-text, ArrowUp/ArrowDown move it between lines; history is navigated only at the text boundaries — ArrowUp at the very start, ArrowDown at the very end.

**Related:** #2473 (planned composer state-model refactor), #3635 (chat components tracker). Happy to open a PR with a fix + tests if this is confirmed as a bug.

### Reproduction

_Recording (astryx.dev docs playground): multi-line draft — pressing ArrowUp mid-text replaces the draft with the previous message instead of moving the caret up a line._

https://github.com/user-attachments/assets/17bb8491-6028-4f49-8ca5-d169b60b2053

**Steps:**
1. Open the `ChatComposerInput` playground on https://astryx.atmeta.com/components/ChatComposerInput — or Storybook → `Core/ChatComposerInput` → **Message History**.
2. Submit a couple of messages (type + Enter) to build history.
3. Type a multi-line draft: `aaa`, Shift+Enter, `bbb`.
4. Place the caret on the first line (or mid-text) and press ArrowUp.
5. **Expected:** the caret moves up a line. **Actual:** the draft is replaced by the previous message; the caret can't move.

### Astryx Version

@astryxdesign/core@0.1.8

### Environment

Chrome 150.0.7871.130 (official build, arm64), macOS 26.5.1 (Tahoe), Apple M5. Keyboard interaction; not AT-specific.

Start with the ChatComposerInput playground or Storybook → Core/ChatComposerInput → Message History and reproduce ArrowUp/ArrowDown behavior in a multi-line draft. Inspect the ChatComposerInput keyboard handling and add regression tests for navigation at text boundaries versus mid-text. Done means caret movement works within multi-line text while history recall remains available at the start and end.

Written by the indexing model from the issue text.