fix(opencode): add namespaced session identity headers - #52370

Issue for this PR

Closes #39912.

Also addresses #39912 (comment). Dev counterpart of the V2 PR #52368.

Type of change

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

What does this PR do?

Adds x-opencode-session-id for the current session's actual ID and x-opencode-parent-session-id for its immediate parent, omitted for roots. Both the legacy provider-aware request builder and the core runner emit them, including custom-provider requests.

Existing headers, cache keys, routing, and configured/plugin header overrides remain unchanged. The current-session header comes before parent headers for LiteLLM's first-match vendor-header scan.

How did you verify your code works?

• 33 tests passed in the legacy LLM suite, including root/child requests on OpenCode and custom providers and preservation of legacy headers.
• 87 tests passed in the core session runner suite.
• bun typecheck passed in packages/opencode and packages/core.
• Changed-file lint passed with warnings. Full-repo lint fails on an existing octal escape in packages/session-ui/src/v2/components/prompt-input/index.tsx:163, unchanged by this PR.

Screenshots / recordings

N/A — request metadata only.

Checklist

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