fix(core): add namespaced session identity headers - #52368

Issue for this PR

Closes #39912.

Also addresses #39912 (comment). This covers V2; the dev counterpart is #52370.

Type of change

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

What does this PR do?

Adds two headers in V2's shared session model-request builder, for all providers:

• x-opencode-session-id: the current session's actual ID, including child sessions.
• x-opencode-parent-session-id: its immediate parent's ID, omitted for root sessions.

Existing headers and prompt-cache keys remain unchanged. Those can carry shared affinity IDs, so the new identity headers deliberately use session identity rather than affinity. The current-session header is emitted before parent headers so LiteLLM's first-match vendor-header scan sees it first.

How did you verify your code works?

• 250 tests passed across session runner, title, compaction, and generation suites.
• Root-session assertions check the exact header set, including omission of the parent header. Child-session assertions verify its own identity, parent linkage, and unchanged parent cache affinity.
• bun run check (lint and workspace typechecks).

Screenshots / recordings

N/A — request metadata only.

Checklist

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