test: cover untested behavioral branches (#44)

[microsasa]

Closes #44

Changes

Adds direct test coverage for three behavioral branches that had no tests:

1. last_resume_time display in report.py

• TestRenderLiveSessions.test_last_resume_time_used_over_start_time — verifies render_live_sessions shows duration from last_resume_time (not start_time) when both are set.
• TestRenderFullSummary.test_active_section_uses_last_resume_time — same guarantee for _render_active_section in the full summary view.

2. _FileChangeHandler debounce in cli.py

• TestFileChangeHandler.test_dispatch_sets_event_on_first_call — first dispatch sets the change event.
• TestFileChangeHandler.test_dispatch_suppresses_within_debounce_window — rapid second call within 2 s is suppressed.
• TestFileChangeHandler.test_dispatch_fires_again_after_debounce_gap — dispatch fires again after > 2 s gap.
• test_stop_observer_none_is_noop — _stop_observer(None) returns without raising.

3. _read_config_model non-string values in parser.py

• test_config_model_integer_returns_none — {"model": 42} → None
• test_config_model_null_returns_none — {"model": null} → None
• test_config_model_list_returns_none — {"model": []} → None

Testing

All 10 new tests follow existing patterns and conventions. No source code changes — tests only.

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[Copilot]

Pull request overview

Adds regression tests to ensure resumed-session runtime is calculated from last_resume_time, improves parser robustness around malformed config.json model fields, and adds coverage for CLI watchdog observer utilities.

Changes:

• Add report rendering tests to verify last_resume_time takes precedence over start_time when computing “running time”.
• Add parser tests ensuring non-string config.json["model"] values result in summary.model is None.
• Add CLI tests for _stop_observer(None) and _FileChangeHandler debounce behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/copilot_usage/test_report.py	New regression tests for “running time” display using last_resume_time in live + full summary views.
tests/copilot_usage/test_parser.py	New tests for config model parsing when model is invalid type (int/null/list).
tests/copilot_usage/test_cli.py	New tests for observer stop guard and file-change debounce behavior.

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