test: add offset assertion and merged-summary verification for incremental parse (#1169)

[microsasa]

Closes #1169

Changes

Strengthens TestSessionCacheMtime.test_unchanged_file_not_reparsed and adds a regression companion test to guard the incremental-merge path in get_all_sessions.

1. Offset assertion (test_unchanged_file_not_reparsed)

assert spy.call_args[0][1] > 0, (
    "expected incremental parse from cached offset, got full reparse from 0"
)

Previously the test used ANY for the offset argument, which would pass even if the incremental path's ev_cached.end_offset > 0 guard were broken and a full reparse from 0 occurred.

2. Merged-summary data verification (test_unchanged_file_not_reparsed)

sess_a = next(s for s in result2 if s.events_path == p1)
assert sess_a.is_active is True
assert sess_a.active_output_tokens == 42

The appended assistant.message event has outputTokens=42. After the incremental merge, the resumed session must reflect these tokens. Previously only session count was checked.

3. Regression test (test_incremental_merge_preserves_cached_events)

A dedicated test that:

• Verifies shutdown model_metrics from the cached prefix survive the merge (would fail if list(ev_cached.events) + new_events were replaced with just new_events)
• Verifies the appended event's outputTokens appear in active_output_tokens
• Verifies user_messages count includes the original user message from cached events

This mirrors the approach used in TestIncrementalCacheLogic.test_incremental_append_uses_cached_prefix in test_vscode_parser.py.

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[Copilot]

Pull request overview

This PR strengthens the test suite coverage around copilot_usage.parser.get_all_sessions incremental parsing by adding assertions that the cached-offset fast path is actually used and that merged session summaries reflect appended event data after an incremental refresh.

Changes:

• Tighten TestSessionCacheMtime.test_unchanged_file_not_reparsed to assert a non-zero incremental offset and validate active_output_tokens after merge.
• Add a new regression test test_incremental_merge_preserves_cached_events to ensure cached-prefix data (e.g., shutdown model_metrics, earlier user_messages) is preserved when new events are appended.