Consolidate duration formatting functions in report.py (#243)

[microsasa]

Closes #243

Changes

Extracted a single _format_timedelta(td: timedelta) -> str core helper that encapsulates the hours/minutes/seconds decomposition and formatting logic. The three existing duration-formatting functions now delegate to it:

• format_duration(ms) → _format_timedelta(timedelta(milliseconds=ms))
• _format_elapsed_since(start) → _format_timedelta(now - ensure_aware(start))
• _format_detail_duration(start, end) → guards for None, then _format_timedelta(end - start)

The format_duration public API signature is unchanged.

Behavior change

The unified helper omits zero-value units (consistent with how format_duration already worked):

• 60s → "1m" (was "1m 0s" in _format_detail_duration)
• 3600s → "1h" (was "1h 0m" in _format_detail_duration)
• Zero elapsed → "0s" (was "0m 0s" in _format_elapsed_since)

Tests

• Added 9 direct unit tests for _format_timedelta covering zero, negative, seconds-only, exact minute, exact hour, full h/m/s, and large durations
• Updated existing test expectations to match the now-consistent formatting
• All 522 tests pass, 98.74% coverage

Generated by Issue Implementer · ◷

[Copilot]

Pull request overview

This PR consolidates the duration formatting logic in copilot_usage.report by introducing a single core _format_timedelta() helper and having existing duration-formatting helpers delegate to it, ensuring consistent output across the CLI report renderers.

Changes:

• Added _format_timedelta(td: timedelta) -> str as the shared duration formatter (clamps negatives to 0s, omits zero-value units).
• Updated format_duration(), _format_elapsed_since(), and _format_detail_duration() to delegate to _format_timedelta().
• Updated boundary expectations and added direct unit tests for _format_timedelta.

Reviewed changes

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

File	Description
src/copilot_usage/report.py	Introduces _format_timedelta() and routes existing duration formatting helpers through it for consistent formatting.
tests/copilot_usage/test_report.py	Updates expectations for zero/minute/hour boundaries and adds focused unit coverage for _format_timedelta().

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[github-actions]

Quality Gate: AUTO-APPROVED

Low-impact refactoring that consolidates three duration-formatting functions into a single _format_timedelta helper. No public API changes, no data model changes, no security concerns. 9 new unit tests with good edge-case coverage. All 8 CI checks green. Auto-approving for merge.