test(git): remove network and git layout dependencies

[CAOShurong]

Description of the PR

This removes two sources of nondeterminism from the Git collector test:

• build a two-document Git fixture in a temporary local repository instead of cloning GitHub;
• assert only the known fixture documents instead of depending on the number of .git implementation files;
• stop the polling case as soon as both fixture documents arrive, with a 10-second safety timeout.

A recent integration run (32978753396, job 98209847685) timed out after collecting 0 files. On the current base, the polling subtest also failed 20/20 local runs because the checkout contained 10 files while the test hard-coded 9. The updated test passed 50 consecutive runs.

This addresses one source of the integration-test flakiness tracked in #2930. It intentionally does not change production collector behavior.

Validation

• go test ./pkg/handler/collector/git -count=50
• git diff HEAD^ HEAD --check

The broader collector suite still has pre-existing Windows-only path-separator failures in the file collector tests; those reproduce unchanged on the base commit and are outside this PR.

Implementation and investigation were assisted by OpenAI Codex. I reviewed the diff and ran the validation above.

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- generated using git commit -s.
• All new changes are covered by tests
• GraphQL schema is unchanged
• OpenAPI spec is unchanged
• ent schema is unchanged
• collectsub protobuf is unchanged
• All CI checks are passing (awaiting CI)
• There are no dependent PRs