⚡ Optimize repeated repository tree fetching

[myaple]

⚡ Performance Improvement: Cache Repository Tree Fetching

💡 What

Implemented a caching mechanism for get_repository_tree in the GitlabApiClient. The cache uses DashMap for thread-safe concurrent access and has a Time-To-Live (TTL) of 5 minutes.

🎯 Why

The repository file tree was being fetched repeatedly (via RepoContextExtractor::get_all_source_files) during context extraction for issues and merge requests. Since the file tree structure rarely changes drastically within a short period, repeated fetching was causing unnecessary latency and API rate limit consumption.

📊 Measured Improvement

• Baseline: get_repository_tree made an API call (or multiple for pagination) every time it was invoked. Verified by test test_get_repository_tree_cache_behavior which initially expected 2 calls for 2 invocations.
• Optimization: With the cache, subsequent calls within the 5-minute window return the cached file list instantly without hitting the GitLab API. Verified by updating the test to expect only 1 API call for 2 invocations.

Implementation Details

• Added repo_tree_cache: DashMap<i64, (Vec<String>, Instant)> to GitlabApiClient.
• Initialized cache in GitlabApiClient::new.
• Updated get_repository_tree to check cache before making request.
• Added src/tests/gitlab_cache_test.rs covering the caching logic.

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PR created automatically by Jules for task 3666510817023256830 started by @myaple

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