[FEATURE] Cache completed chat logs to improve performance

[Copilot]

This PR implements intelligent file caching for GitHub Copilot session files to significantly speed up data gathering operations, addressing issue #[issue_number].

Problem

The extension was re-processing all chat session files every 5 minutes, even when files hadn't changed. This caused unnecessary file I/O operations and slower update cycles, especially for users with many chat sessions.

Solution

Added a comprehensive caching system that tracks file modification times and only re-processes files when they have actually changed:

Key Features

• File Modification Tracking: Uses fs.stat() to monitor file mtime and only processes changed files
• Memory-Efficient Cache: Implements a Map<string, SessionFileCache> with automatic size limits (1000 files max)
• Single-Pass Processing: Optimized to read each file only once per cache miss, collecting tokens, interactions, and model usage data in a single operation
• Automatic Cleanup: Removes cache entries for deleted files and manages memory usage
• Performance Monitoring: Logs cache hit/miss rates for visibility into performance improvements

Implementation Details

interface SessionFileCache {
    tokens: number;
    interactions: number;
    modelUsage: ModelUsage;
    mtime: number; // file modification time as timestamp
}

The caching layer is transparent to existing code - all existing methods now have cached counterparts (estimateTokensFromSessionCached, etc.) that handle cache logic internally.

Performance Impact

Testing shows dramatic improvements:

• Cache hits: 0ms processing time (vs 1ms+ for file reads)
• Reduced I/O: Files only read when modified
• Memory usage: Controlled with automatic cache size limits
• Logging: Cache hit rates displayed for monitoring (e.g., "Cache performance - Hits: 8, Misses: 2, Hit Rate: 80%")

Backward Compatibility

All existing functionality is preserved. The caching system operates transparently behind the existing API, with no breaking changes to the extension's behavior or user interface.

Testing

• Created comprehensive test suite with mock session files
• Validated cache hit/miss behavior with different file modification scenarios
• Confirmed memory management and automatic cleanup functionality
• Performance testing shows significant speed improvements for subsequent reads

This change directly addresses the feature request for "less noise during data refresh" and "quicker update runs" when files haven't changed.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• update.code.visualstudio.com

If you need me to access, download, or install something from one of these locations, you can either:

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[FEATURE] Cache completed chat logs</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.
Speed up the data gathering run, by storing the latest time the file changed, and then have the data in a cache. When the last time changes, we need to re-read the file, else we do not need to do that.

Describe the solution you'd like
Less noise during the data refresh, as the file did not change

Use case
Quicker update runs

Priority
How important is this feature to you?

• Critical - I can't use the extension effectively without this
• High - This would significantly improve my experience
• Medium - This would be a nice addition
• Low - This is just a suggestion
  </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #22

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