fix(opencode): declare session http errors

[TimothyVang]

Summary

• declare explicit session.list BadRequest and session.diff BadRequest/NotFound contracts
• keep SessionSummary.diff missing-session failures on the Effect error channel
• map diff NotFound to the declared legacy API not-found body at the HTTP boundary

Verification

• bun test test/session/summary.test.ts test/server/httpapi-session.test.ts test/server/session-diff-missing-patch.test.ts test/server/httpapi-public-openapi.test.ts test/server/httpapi-query-schema-drift.test.ts
• bun run typecheck
• bun run test:httpapi
• bun run check:generated (packages/client)
• umask 022 && bun test
• git diff --check

Note: a default shell umask of 0002 caused the existing test/tool/write.test.ts mode assertion to see 0664 instead of 0644 on the first full-suite run; rerunning the full suite with umask 022 passed.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, add credits to your account and enable them for code reviews in your settings.

[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]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.