fix: remove dead child_ids field from _VSCodeDiscoveryCache (#1174)

[microsasa]

Summary

Remove the unused child_ids field from _VSCodeDiscoveryCache which was stored on every cache miss but never consulted on cache hits, causing silent memory overhead proportional to the number of session directories.

Changes

src/copilot_usage/vscode_parser.py:

• Removed child_ids: _ChildIds from _VSCodeDiscoveryCache frozen dataclass
• Removed child_ids=child_ids from the cache construction in _cached_discover_vscode_logs
• Updated dataclass docstring to remove mention of child_ids being retained
• Kept child_ids as a local variable where it's used (no semantic change)
• Kept _ChildIds type alias (still used by _scan_child_ids and _newest_child_from_ids)

tests/copilot_usage/test_vscode_parser.py:

• Removed child_ids=cached.child_ids kwarg from test_cache_invalidated_on_root_mtime_change
• Replaced assert cached.child_ids == _scan_child_ids(tmp_path) with assertions on newest_child_path and newest_child_id fields
• Removed child_ids=frozenset() from TestVSCodeDiscoveryCacheIsFrozen

Closes #1174

Warning

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Generated by Issue Implementer · ● 9.9M · ◷

[Copilot]

Pull request overview

Removes an unused _VSCodeDiscoveryCache.child_ids field from the VS Code log discovery cache to eliminate unnecessary per-entry memory overhead while keeping cache-hit behavior unchanged.

Changes:

• Dropped the child_ids field from _VSCodeDiscoveryCache and stopped persisting it in _VSCODE_DISCOVERY_CACHE.
• Updated _VSCodeDiscoveryCache docstring to reflect the simplified cached state.
• Adjusted unit tests to stop constructing/asserting against the removed field and validate the remaining sentinel fields instead.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/copilot_usage/vscode_parser.py	Removes the dead cached child_ids field and updates cache population + docstring accordingly.
tests/copilot_usage/test_vscode_parser.py	Updates cache-related tests to match the new dataclass shape and assert on newest_child_* instead of child_ids.