test: exercise MockKestraApiServer so the testFixtures kernel doesn't ship untested

## Summary

The `testFixtures` source set (`io.kestra.plugin.git.shared.testkit`) compiles but no test **in this repo** exercises it, so `MockKestraApiServer` — the in-process mock Kestra API server both plugins' suites depend on — ships without any coverage of its own. A regression in the mock is only caught downstream, in the consuming plugins' CI, after a lib bump.

## Context

Flagged in the PR #1 review ([review](https://github.com/kestra-io/plugin-git-lib/pull/1#pullrequestreview-5165435229)).

## Tasks

- [ ] Add a test in `plugin-git-lib` that boots `MockKestraApiServer` and asserts its core endpoints/behaviour (the surface the consumers rely on).
- [ ] Wire it so it runs under `./gradlew test` in this repo's CI.

## Acceptance criteria

- [ ] `MockKestraApiServer` has direct coverage in `plugin-git-lib`'s own test run.
- [ ] A breaking change to the mock fails this repo's CI, not only the downstream plugins'.

Locate MockKestraApiServer in the testFixtures source set and review the endpoints and behaviour used by the consuming plugin suites. Add direct coverage for that core surface, then run ./gradlew test to verify the test executes in this repository's CI and fails when the mock breaks.

Written by the indexing model from the issue text.