test: add git config integration tests

[patrick-stephens]

Resolves #161 by adding integration tests for the git_config plugin.

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Summary by cubic

Add integration tests for the git_config plugin and pass the current PR SHA to test jobs so the plugin polls the right ref. Implements the coverage requested in #161.

• New Tests

  • BATS test to validate git_config startup, repo polling, commit detection, and switching to the updated config.
  • Adds initial and updated Fluent Bit configs used by the test; updated config logs the current GIT_SHA.
  • Verifies custom plugins are discoverable: git_config, log_sampling, and dedupe.

• CI

  • Sets GIT_SHA in test workflows to ${{ github.event.pull_request.head.sha || github.sha }}.
  • run-bats.sh reads and logs GIT_SHA to drive the tests.

^{Written for commit 9ae46e6. Summary will update on new commits.}