feat: Add stacked pull request endpoints

[Bortlesboat]

Fixes #4435

Add typed PullRequestsService support for GitHub's five stacked pull request
REST endpoints:

• list repository stacks, optionally filtered by pull request;
• create and get a stack;
• append pull requests to a stack; and
• unstack the remaining unmerged pull requests.

The change keeps pull-request membership metadata separate from the endpoint
stack resource, adds typed request and list-option types, handles both
updated-stack and 204 No Content unstack responses, and regenerates accessors
and iterators. Request methods,
paths, query parameters, bodies, response decoding, and failure paths have
focused coverage.

API contract: https://docs.github.com/en/rest/pulls/stacks?apiVersion=2022-11-28

Validation:

• script/fmt.sh
• script/lint.sh across all 12 modules
• script/test.sh -covermode atomic ./... across all 12 modules
• git diff --check

The local Go configuration has CGO disabled, so a race build was not available;
the full non-race suite passed.

AI assistance: OpenAI Codex helped draft the API bindings, tests, generated-file
workflow, and PR description. I reviewed the rendered diff and validation
evidence, understand every submitted line, and take responsibility for the
contribution and review follow-up.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.56%. Comparing base (4a79475) to head (295dd70).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4436   +/-   ##
=======================================
  Coverage   97.55%   97.56%           
=======================================
  Files         194      195    +1     
  Lines       19892    19962   +70     
=======================================
+ Hits        19406    19476   +70     
  Misses        268      268           
  Partials      218      218           

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[gmlewis]

Thank you, @Bortlesboat!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra

[Bortlesboat]

All three review items are addressed in signed commit 295dd70, and their threads are resolved. Local script/test.sh -covermode atomic ./..., script/lint.sh, script/fmt.sh, and script/generate.sh --check all pass. GitHub initially held the new tests, linter, and workflow-security runs at action_required; a maintainer approved them, and the full current upstream matrix is now green.