fix(desktop): preserve agent mention boundary in packaged WebKit

[cmyk]

Summary

Selecting a managed-agent mention in a packaged macOS build could leave ProseMirror's model and DOM selection correctly positioned after the trailing ASCII space, while WKWebView's next native text insertion crossed back into the decorated mention. Typing immediately after autocomplete could therefore change @Reinhold into @Reinholdtest, remove the agent decoration, and silently drop the intended recipient tag.

This change:

• records authoritative metadata for the agent-prefix and agent-label decoration segments and exposes a query for the exact decorated range adjacent to the caret;
• intercepts only ordinary text insertion at a collapsed selection after a U+0020 separator whose preceding range is authoritatively decorated as an agent mention;
• prevents the faulty native mutation and dispatches one ProseMirror transaction that preserves the separator and places the selection after the inserted character;
• leaves undecorated matching prose, marked text, split text nodes, composition/IME, paste, deletion, and unrelated typing on their existing paths;
• adds focused helper and mounted production-wiring coverage;
• adds a Chromium smoke test for the cross-browser composer separator-geometry invariant, while keeping native packaged-WKWebView acceptance as separate evidence.

Related issue

No exact issue found.

Related but distinct open work:

• fix(desktop): snap caret out of agent mention chips #2712 handles explicit caret placement and editing inside an existing mention chip.
• fix(desktop): keep the space after a mention through the markdown parse #2924 restores trailing whitespace lost during Markdown parsing.
• fix(desktop): keep trailing space after persistent agent mentions #5021 restores trailing whitespace after persistent-agent post-send content restoration.

This PR addresses the packaged-WKWebView native insertion immediately after autocomplete, where the model selection and separator were already correct.

Testing

Automated verification at exact head 51cafdb0d55168aaffafc016bdf0acc585d780d6:

• focused mention-boundary, decoration, and mounted production-wiring tests: 34 passed, 0 failed;
• full Desktop unit suite: 4,491 passed, 0 failed;
• E2E frontend build: passed, 4,697 modules transformed;
• Chromium composer separator-geometry invariant: 1 passed, 0 failed;
• TypeScript typecheck: passed;
• canonical frontend check: passed across 1,930 files; one unrelated pre-existing warning and two unrelated pre-existing infos remain;
• changed-file Biome and repository helper checks: passed;
• git diff --check: passed;
• independent read-only review: approved.

Manual acceptance in an isolated packaged macOS candidate:

1. Select @Reinhold from autocomplete.
2. Type immediately without adding a space manually.
3. Before the fix, the caret appeared inside the mention and native insertion could extend the decorated name.
4. With the fix, typing test produces @Reinhold test; the mention remains decorated, the separator remains visible, and the caret ends after the inserted text.

Before

After