Add atomic head SHA pinning to merge_pull_request

[jcosta1970]

Problem

merge_pull_request currently has no way for a caller to atomically assert
which pull-request HEAD is being merged.

An agent can inspect a pull request, validate CI/review readiness, and then
call merge_pull_request, but the PR HEAD may change between those steps.
Without a compare-and-swap condition on the merge call, that creates a TOCTOU
gap.

The GitHub REST merge endpoint already supports the optional sha field for
this purpose, and go-github already exposes it as
PullRequestOptions.SHA. The MCP tool currently does not expose or forward
that capability.

Proposed change

Add an optional expectedHeadSha input to merge_pull_request and pass it
directly to github.PullRequestOptions.SHA.

This follows the existing expectedHeadSha naming used by
update_pull_request_branch.

When supplied:

• the exact expected HEAD SHA is sent to GitHub as the REST sha field;
• a changed HEAD is rejected by GitHub rather than merging a different commit;
• no retry or permissive fallback is introduced.

When omitted, behavior remains backward compatible because the underlying
sha field is optional/omitted.

Scope

No authentication, credential, logging, permission, or unrelated tool changes.

[leuasseurfarrelds247-arch]

Problem

merge_pull_request currently has no way for a caller to atomically assert
which pull-request HEAD is being merged.

An agent can inspect a pull request, validate CI/review readiness, and then
call merge_pull_request, but the PR HEAD may change between those steps.
Without a compare-and-swap condition on the merge call, that creates a TOCTOU
gap.

The GitHub REST merge endpoint already supports the optional sha field for
this purpose, and go-github already exposes it as
PullRequestOptions.SHA. The MCP tool currently does not expose or forward
that capability.

Proposed change

Add an optional expectedHeadSha input to merge_pull_request and pass it
directly to github.PullRequestOptions.SHA.

This follows the existing expectedHeadSha naming used by
update_pull_request_branch.

When supplied:

• the exact expected HEAD SHA is sent to GitHub as the REST sha field;
• a changed HEAD is rejected by GitHub rather than merging a different commit;
• no retry or permissive fallback is introduced.

When omitted, behavior remains backward compatible because the underlying
sha field is optional/omitted.

Scope

No authentication, credential, logging, permission, or unrelated tool changes.

[github/github-mcp-server] Erro spawn EINVAL ao tentar usar a tool execute_test_plan (Issue #3180) ([github/github-mcp-server] Erro spawn EINVAL ao tentar usar a tool execute_test_plan (Issue #3180))