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 11 modules
• script/test.sh -covermode atomic ./... across all 11 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 98.54%. Comparing base (3a439fa) to head (fd02503).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4436   +/-   ##
=======================================
  Coverage   98.53%   98.54%           
=======================================
  Files         195      196    +1     
  Lines       17848    17914   +66     
=======================================
+ Hits        17587    17653   +66     
  Misses        261      261           

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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.

[Bortlesboat]

Thanks, both differences check out against the OpenAPI description. Pushed 5ad53ca.

base is {ref} only on all five stack endpoints, so it no longer reuses
PullRequestStackBase — that type stays as-is for PullRequest.Stack, where
base really is {ref, sha}. The stack endpoints now use a new
PullRequestStackRef.

On the list endpoint: the difference is in pull_requests. List items are
{number, state, draft, merged_at, head{ref, sha}}, while create/get/add/unstack
return Pull Request Stack Pull Request, which is Pull Request Minimal
(id, number, url, head/base with a nested repo) plus node_id,
title, state, merged_at, draft, html_url, and user. So ListStacks
now returns []*PullRequestStackMinimal and the other four return
*PullRequestStackDetails. The head/base shapes differ too (list has no
repo), hence the separate branch types.

Also dropped omitempty and the pointers on the required fields per your other
comment. The old test fixture was sending the list-shaped pull_requests for
every endpoint, so there are two fixtures now.

[Not-Dhananjay-Mishra]

A few struct name suggestions.

[Bortlesboat]

Took the stutter fixes in 7b58560, but kept the PullRequestStack prefix
rather than dropping to bare Stack*:

• PullRequestStackPullRequest -> PullRequestStackEntry
• PullRequestStackMinimalPullRequest -> PullRequestStackMinimalEntry
• PullRequestStackMinimalBranch -> PullRequestStackMinimalHead

Reason for not using StackPullRequest/StackMinimalPullRequest: every other
stack type in the package is prefixed (PullRequestStack, PullRequestStackBase,
PullRequestStackDetails, PullRequestStackMinimal, ...), so those two would be
the only unprefixed ones in the group. Entry drops the repetition without
splitting the naming. Happy to go bare if you'd still rather.

I went with PullRequestStackMinimalHead over PullRequestStackHead because the
detailed responses have a head too, in PullRequestStackBranch — an unqualified
PullRequestStackHead would be ambiguous about which one it is.

On the two request types, I'd lean toward leaving them: Create<Resource>Request
is what the package mostly does (19 types — CreateIssueRequest,
CreateGistCommentRequest, CreateHostedRunnerRequest, ...) against 4 with a
service prefix, and those 4 are prefixed by service name
(ActionsCreateVariableRequest, SecretScanningCreateCustomPatternsRequest). If
you do want the service-prefixed form here, PullRequestsCreateStackRequest and
PullRequestsAddToStackRequest would at least match each other — the suggestions
as written are singular for one and plural for the other.

[Bortlesboat]

Refreshed against current master in 064c6e7. The two request types now use the suggested names, and each endpoint test contains its response and expected-value fixtures directly. Accessors and iterators are regenerated.

Formatting, generated-file freshness, all 11 modules' non-race tests, and the custom lint checks pass. The focused stack endpoint tests also pass after replacing the copied pointer helpers with the current Go idiom. Local CGO remains disabled, so race coverage is left to CI.

The separate endpoint-resource and embedded pull-request membership types are preserved for the schema distinction discussed above.