fix(opencode): identify agent creation requests

[Dante-dan]

Issue for this PR

Closes #52192

Type of change

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

What does this PR do?

opencode agent create calls Agent.generate outside a saved session. Its one-shot request omitted x-opencode-session, so OpenCode Go rejected it before it could create an agent. This adds a temporary session ID to OpenCode provider generation requests; it does not create a stored session or change requests to other providers.

This is the CLI generation path. #47473 covers normal session-runner requests, while the merged #48358 covers one-shot generation on the separate v2 branch.

How did you verify your code works?

• A local OpenAI-compatible HTTP fixture rejects a request without the header. The new Agent.generate regression reaches that fixture, returns the generated agent, and confirms a ses_ header was sent. bun test test/agent/agent.test.ts --test-name-pattern 'agent generation identifies' passed with Bun 1.4.2.
• tsgo --noEmit -p packages/opencode/tsconfig.json, Prettier check, and git diff --check passed. Targeted oxlint reported no errors and existing warnings in the surrounding test file.
• The full agent test file had 41 passes and three failures while typecheck ran concurrently: two 5-second timeouts (including the new test before its timeout was raised) and an FSEvents startup error. The new test passed when rerun after the timeout change.

Screenshots / recordings

N/A — CLI request fix.

Checklist

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

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