fix(session-ui): truncate long reasoning headings instead of overflowing

[CannonRS]

Issue for this PR

Closes #48007

Type of change

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

What does this PR do?

Long reasoning headings in the session-turn-thinking row overflow the timeline
to the right: the heading is a flex child with min-width: 0 but no overflow
handling, and TextReveal renders with white-space: nowrap.

The change clips the heading to the available row width:

• session-turn.css: .session-turn-thinking-heading gains
  overflow: hidden; white-space: nowrap; text-overflow: ellipsis.
• session-turn.tsx / message-timeline.tsx: both thinking rows pass truncate
  to TextReveal, which the component already supports (data-truncate sets the
  track width to 100% and applies ellipsis). No new component surface is added.

How did you verify your code works?

• TextReveal.truncate already exists upstream (props.truncate + data-truncate
  CSS), so the change is purely wiring + one CSS rule.
• Typecheck in packages/session-ui in the fork run is green; the worktree
  typecheck is currently blocked by an environment-level vite/client.d.ts
  parse error from the worktree bun install (unrelated to this change — the
  same files typecheck in the fork where the identical code is committed).
• CSS git diff --check clean; diff is +5/-1 across three files.

Screenshots / recordings

N/A — visual-only clipping fix, no behavioural change outside the clipped heading.
A recording can be added on request.

Fork

• Source fork: https://github.com/mQorva/OpenCode
• Diff against this fork's dev: mQorva/OpenCode@dev...thinking-heading-truncate

Checklist

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