fix(session-ui): preserve caret after prompt editor rebuild - #42297

Issue for this PR

Fixes #42335

Related to #37037 / #36210 (caret jumps around in the prompt input while the
session updates). This PR fixes the variant I could reproduce and verify on the
current v2 composer: inserting a @mention mid-text on the session page
collapses the caret to the END of the prompt instead of keeping it right after
the mention. I could not reproduce the streaming-output case from #37037 on
current dev, so I only claim the mention-insertion scenario here.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

renderPromptInputV2Editor rebuilds the editor DOM (replaceChildren) whenever
the prompt store changes outside a local input (mention insertion, history
navigation, followup edit). After the rebuild it always collapsed the caret to
the end of the editor, ignoring the cursor position stored in the prompt store.
Reproduced on the session page: after inserting @src/index.ts mid-text, the
caret lands at offset 32 (end of a 32-char prompt) instead of 21 (right after
the mention).

Changes in packages/session-ui/src/v2/components/prompt-input:

• interaction.ts: expose cursor() on the controller so the editor can read
  the authoritative stored caret position.
• index.tsx: restore the caret from the stored cursor after a rebuild, falling
  back to the pre-rebuild DOM caret when no cursor is stored. Mentions are
  uneditable spans that still occupy their text length, so
  setPromptInputV2Cursor places the caret before/after a mention, never inside
  it.

How did you verify your code works?

• New e2e regression test packages/app/e2e/regression/prompt-caret-after-mention.spec.ts:
  types "please fix the bug", moves the caret to offset 7, inserts
  @src/index.ts mid-text on a session page, and asserts the caret lands at
  offset 21. It failed before the fix (caret at 32) and passes after.
  Run with: bunx playwright test e2e/regression/prompt-caret-after-mention.spec.ts
• bun run typecheck in packages/session-ui passes.
• bun test src in packages/session-ui: 83 tests pass.
• Existing prompt e2e (prompt-input-v2-command-draft) still passes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR