fix(serve): resolve missing revisions via on-demand refetch

[tinder-maxwellelliott]

Problem

The bazel-diff serve query service returned an opaque 400 for commits it should have been able to serve:

Error: bazel-diff server returned 400: { "error": "git error: JGit checkout 9ea41ff… failed: Missing unknown 9ea41ff…" }

Two compounding causes:

1. resolveSha didn't verify the commit exists. JGit's Repository.resolve("<40-hex>") parses the hex into an ObjectId without an object-DB lookup, and git rev-parse <full-sha> echoes any well-formed SHA straight back. So a full SHA that isn't in the local clone "resolved" successfully, and the real failure (MissingObjectException) was deferred to the later checkout — surfacing as a confusing checkout error.
2. The service only fetched once, at startup. Any commit that landed on the remote afterwards (or lived on a ref the default refspec doesn't cover) could never be served without a restart.

Fix

• Validate object existence in resolveSha for both git engines, raising a new retryable MissingRevisionException:
  • JGit: RevWalk.parseCommit(objectId) (also peels annotated tags to their commit).
  • Subprocess: rev-parse --verify <rev>^{commit} instead of a bare rev-parse.
• On-demand refetch + retry in ImpactedTargetsService.resolveBoth: if either revision is missing, run one git fetch and retry. Serialized and double-checked via a fetchLock, so a burst of requests for the same just-landed commit triggers at most one fetch.

Resulting behavior

Scenario	Before	After
Commit landed after startup	400 … Missing unknown …	Fetched on demand and served
Genuinely unknown SHA / uncovered ref	400 (opaque)	400 after one refetch, clear message: revision '<x>' is missing from the local clone
Named ref at a stale-but-present tip	unchanged	unchanged (fetch-once staleness is a separate concern, out of scope)

The --gitEngine=subprocess path had the same latent bug and is fixed the same way.

Tests

All green via bazel test:

• JGitClientTest — absent full SHA → MissingRevisionException; end-to-end repro (resolveShaSeesCommitOnlyAfterFetch: a commit lands in an origin after clone, is rejected locally, then resolves post-fetch()); non-commit object → hard error.
• GitClientTest (subprocess) — absent full SHA → MissingRevisionException.
• ImpactedTargetsServiceTest — refetch-then-succeeds; double-check skips the redundant fetch; persistent miss propagates after exactly one fetch.

Ran //cli:{GitClientTest,JGitClientTest,ImpactedTargetsServiceTest,BazelDiffServerTest,HashServiceTest,ServeCommandTest} (6/6 pass) and applied //cli/format (ktfmt).

Notes for reviewers

• GitClientException is now open so MissingRevisionException can extend it; the HTTP layer's existing catch (GitClientException) → 400 mapping still covers it.
• Follow-up to the serve service (feat: add bazel-diff serve HTTP query service (#29) #393 / issue RFC: Bazel Query Service #29).

🤖 Generated with Claude Code