feat(sdk): detect PR context on Jenkins multibranch builds (#234)

[xping-admin]

Closes #234.

Jenkins Branch Source PR builds now carry a PullRequestContext.

How the PR head commit is found

No Jenkins env var holds the PR head under the merge strategy (the default). Jenkins checks out the PR head and merges the target into it, so:

• Read refs/remotes/origin/$BRANCH_NAME from $WORKSPACE/.git (loose ref, then packed-refs).
• Use it only if it equals GIT_COMMIT (head strategy, or the merge fast-forwarded) or GIT_COMMIT's first parent (merge strategy).
• Anything else returns null. For example, the ref moved past the built commit, or .git is a file.

Changes

• JenkinsPullRequestDetector: gated on CHANGE_ID. Platform, owner and name come from CHANGE_URL: github.com, gitlab.com (nested groups), and Azure DevOps Services (dev.azure.com and *.visualstudio.com). Owner/name match the Azure Pipelines and GitLab detectors. If the PR number in the URL ≠ CHANGE_ID, it returns null.
• GitRepositoryReader: minimal read-only .git access. It reads refs and the first parent of a loose commit (zlib via DeflateStream). There's no pack parsing: a commit packed by git gc --auto (long-lived workspaces) falls back to git rev-parse <sha>^1 with a 5 s timeout, and returns null without git on PATH. It validates ref names and SHAs before building file paths.
• EnvironmentDetector: CI.IsPullRequest uses the shared IsPullRequestBuild predicate. GHPRB is still flagged.
• Registered in AddXpingPullRequest.
• Docs: configuration-reference.md and the Jenkins section and commit table of ci-cd-setup.md.

Tests

• JenkinsPullRequestDetectorTests:
  • each host, unknown hosts, bad CHANGE_ID, URL/ID mismatch, each required variable missing
  • head / merge / moved-ref / unreadable cases
  • parity with the Azure Pipelines and GitLab detectors
• GitRepositoryReaderTests: run against real repos built with the git CLI in a temp dir. They cover loose and packed refs, a --no-ff merge's first parent, root and packed objects, a worktree .git file, and unsafe inputs.

Not verified

Not yet run on a real Jenkins. If GIT_COMMIT or the PR-n / MR-n ref names differ from what the plugin source shows, the detector returns null rather than a wrong commit.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 99.09502% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...vices/PullRequest/Internals/GitRepositoryReader.cs	98.09%	2 Missing ⚠️

Files with missing lines	Coverage Δ
...ore/Extensions/XpingServiceCollectionExtensions.cs	96.28% <100.00%> (+0.02%)	⬆️
...vices/Environment/Internals/EnvironmentDetector.cs	72.09% <100.00%> (+1.18%)	⬆️
...ullRequest/Internals/JenkinsPullRequestDetector.cs	100.00% <100.00%> (ø)
...vices/PullRequest/Internals/GitRepositoryReader.cs	98.09% <98.09%> (ø)

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.