Refactor file stat handling to always check for modifications, improving accuracy and performance

[rajbos]

This pull request updates the token tracking logic in the CopilotTokenTracker class to always perform a file stat operation when processing session files, instead of relying on cached metadata to skip the stat. This change ensures that file modifications are reliably detected, trading a minor performance cost for improved data accuracy. The cache is still used for reading session data efficiently, but not for skipping file stats.

Key changes:

File stat and cache logic updates:

• Always calls statSessionFile to get up-to-date mtime and size for each session file, instead of using cached values to skip the stat. This improves reliability in detecting file changes. [1] [2] [3] [4]
• Updates cache usage: the cache is now used to validate if session data can be reused based on the latest mtime and size, rather than to skip the stat entirely.
• When retrieving the repository name, now uses the value from the session data (which is validated against the current file) instead of potentially stale cached metadata.