[task] Add GitHub API fixture tests

[phongndo]

Goal

Add test fixtures for GitHub API responses used by the first app-server data methods.

Why this matters

Contributors should be able to develop issue, PR, checks, and review behavior without needing live GitHub access for every test.

Command/API target

Create a focused fixture pattern for recorded or hand-written GitHub REST/GraphQL response payloads used by the Rust mapping layer.

Depends on

• [task] Add GitHub API adapter using gh api #4 for the GitHub adapter boundary.
• [task] Define shared GitHub workflow data types #6 for shared workflow data types.

Out of scope

• Do not add a full HTTP mocking framework unless needed.
• Do not require network access in normal tests.
• Do not build UI tests here.

Definition of done

• Fixture examples cover at least issue, PR, checks, and review-thread shaped payloads.
• Mapping tests can run without live GitHub credentials.
• The fixture pattern is documented enough for later contributors to copy.

Verification

• cargo fmt --all --check
• cargo test --workspace

Contributor notes

This is maintainer-owned until the fixture approach is stable.