feat: add affectedFiles to parsed pull_request event

[hmacr]

Related SER-401

[greptile-apps]

Greptile Summary

This PR adds an affectedFiles field to the parsed pull_request webhook event for both GitHub and Gitea adapters, fetching the list of changed files via the respective REST APIs using a new getPullRequestFiles method on each adapter. The feature is well-tested with both unit and integration tests.

• getPullRequestFiles in GitHub.php uses an unbounded while (true) pagination loop with no maximum page guard, unlike the Gitea implementation which caps at $maxPages = 100. A bug or edge case causing the API to consistently return $perPage items could result in an infinite loop.
• The GitHub implementation also lacks HTTP status code checking: if the API returns a 4xx/5xx error, the error body is silently merged into the file list and affectedFiles degrades to [] without any signal to the caller — inconsistent with the Gitea implementation which explicitly throws.
• Both adapters call getPullRequestFiles unconditionally inside getEvent with no try/catch. Since Gitea's implementation throws on HTTP errors, a malformed payload (e.g. missing number field causing (int)'' → 0) will surface as an uncaught exception and crash event parsing entirely. The same risk exists in GitHub if curl-level errors occur.

Confidence Score: 2/5

• The PR introduces real reliability risks: an infinite loop potential in GitHub pagination and uncaught exceptions that could crash all pull_request webhook processing in both adapters.
• Two P1 issues — a missing max-page guard in the GitHub pagination loop and unguarded exception propagation in both adapters' getEvent — mean the changes could break webhook handling for all pull request events under certain error conditions. The Gitea implementation is more robust but still shares the getEvent exception risk.
• src/VCS/Adapter/Git/GitHub.php (infinite loop risk + missing error handling in getPullRequestFiles) and both adapters' getEvent call sites (unguarded getPullRequestFiles invocation).

Important Files Changed

Filename	Overview
src/VCS/Adapter/Git/GitHub.php	Adds getPullRequestFiles (paginated, but no max-page guard and no HTTP error check) and calls it unconditionally in getEvent for pull_request events — a thrown exception or infinite loop would break all PR webhook processing.
src/VCS/Adapter/Git/Gitea.php	Adds getPullRequestFiles with proper HTTP status checking and a $maxPages guard; however, calling it unconditionally in getEvent without a try/catch means an exception on a malformed payload (e.g. missing number) will crash the entire event parse.
src/VCS/Adapter.php	Adds the abstract getPullRequestFiles method signature to the base adapter — straightforward addition with no issues.
tests/VCS/Adapter/GitHubTest.php	Adds testGetPullRequestFiles and updates testGetEventPullRequest to assert affectedFiles; test assertions look correct and well-targeted.
tests/VCS/Adapter/GiteaTest.php	Rewrites testGetEventPullRequest and testGetEventPullRequestExternal to use real API resources and adds testGetPullRequestFiles; tests are more realistic and now assert affectedFiles correctly.
tests/VCS/Base.php	Adds testGetPullRequestFiles as a required abstract test method — no issues.

_{Reviews (1): Last reviewed commit: "feat: add affectedFiles to parsed pull..." | Re-trigger Greptile}