fix(otel): propagate incoming trace context for HTTP requests

[Betula-L]

Issue for this PR

Fixes #18801
Related to #219
Related to #12142

Type of change

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

What does this PR do?

This propagates incoming W3C trace context at the HTTP boundary.

If a request includes traceparent / tracestate, the server now extracts that context and runs request handling inside it. This lets request traces attach to an upstream trace instead of starting disconnected traces at the server boundary.

I kept this scoped to standard OTel/W3C propagation in core. It does not add a vendor-specific tracing backend.

How did you verify your code works?

• bun test test/server/trace-context.test.ts
• bun run typecheck
• verified against a real sandbox flow with Langfuse via the existing plugin/export path

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

Hey! Your PR title trace: propagate incoming trace context for HTTP requests doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍