[pei711]

Summary

Propagate GitHub API errors from branch, commit, and tag resource completions.

Why

Failed lookups currently appear as successful empty results, hiding rate-limit, access, and transport errors. Fixes #3581.

What changed

• Return errors from ListBranches, ListCommits, and ListTags.
• Add mocked 403 regression coverage for all three completion paths.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Resource completions now report GitHub lookup failures instead of successful empty values. No tool schema changed.

Prompts tested (tool changes only)

Not applicable; this change affects resource completions, not tool calls.

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with go test -v ./...

Docs

• Not needed
• Updated (README / docs / examples)

AI assistance

OpenAI Codex assisted with the implementation and regression test.

[pei711]

Closing this duplicate in favor of #3583, which was opened first and covers the same resource completion API errors in the same source and test files. My local tests and lint passed.