ref(vcs): Improve find_base_sha debuggability

[szokeasaurusrex]

Rather than emitting debug! logs (which we don't show in tests) within the find_base_sha and extract_pr_base_sha_from_event function, improve these methods by returning proper error types, which include the underlying error as a cause.

The calling code is now responsible for debug-logging these errors, which is still desired, as we expect these functions to error when run outside of a GitHub Actions environment. But, now we will have more information available when debugging test failures, and the debug logs at runtime will also likely be more detailed.

---

Note

Refactors base SHA discovery to return Result<Option> with contextual errors and updates caller to debug-log failures.

• VCS utils (src/utils/vcs.rs):
  • Change find_base_sha to Result<Option<String>>; add contextual errors when reading/parsing GITHUB_EVENT_PATH.
  • Make extract_pr_base_sha_from_event return Result<Option<String>> with JSON parse context.
  • Import Context and propagate errors instead of silent debug logs.
  • Update tests to expect Option on success paths and errors on malformed JSON/missing env.
• Build upload (src/commands/build/upload.rs):
  • Update base SHA resolution to handle Result<Option<...>>; add inspect_err debug logging and flatten Option.

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[runningcode]

TIL inspect_err. nice!

[lcian]

LGTM