[aw][code health] `render_session_detail` uses `datetime.now(tz=UTC)` as `session_start` when `events[0].timestamp` is None, sil [Content truncated due to length]

[microsasa]

Root Cause

In render_session_detail (src/copilot_usage/render_detail.py), session_start is computed as:

session_start = (
    ensure_aware(summary.start_time) if summary.start_time is not None
    else (
        ensure_aware(events[0].timestamp) if events and events[0].timestamp is not None
        else datetime.now(tz=UTC)
    )
)
```

When `summary.start_time is None` **and** `events[0].timestamp is None`, the fallback is `datetime.now(tz=UTC)`. If any later event in the list has a timestamp from the past, its delta is:

```
delta = ensure_aware(ev.timestamp) - datetime.now(tz=UTC)  →  negative

_format_relative_time computes max(int(delta.total_seconds()), 0), clamping all negative deltas to 0. Every event with a past timestamp therefore displays +0:00 instead of the actual relative offset from the earliest known event.

This is a silent data-loss bug — the timing column renders incorrect values with no error or warning.

Affected Code

• src/copilot_usage/render_detail.py — the session_start ternary (currently only checks events[0])
• src/copilot_usage/render_detail.py — _format_relative_time (correct, no change needed)

Reproduction Scenario

Session with:

• summary.start_time = None
• events[0].timestamp = None (e.g., a synthetic/placeholder event)
• events[1].timestamp = <some past datetime>
• events[2].timestamp = <some past datetime + 5 min>

Current behaviour: both events[1] and events[2] show +0:00 in the time column.

Expected behaviour: events[1] shows +0:00 (it anchors the view), events[2] shows +5:00.

Fix

Scan all events for the first non-None timestamp rather than only checking events[0]:

session_start = (
    ensure_aware(summary.start_time)
    if summary.start_time is not None
    else next(
        (ensure_aware(ev.timestamp) for ev in events if ev.timestamp is not None),
        datetime.now(tz=UTC),
    )
)

Testing Requirement

Add a regression test to tests/copilot_usage/test_render_detail.py covering the mixed-timestamp case:

• start_time=None
• events[0].timestamp = None
• events[1].timestamp = T (a fixed past datetime)
• events[2].timestamp = T + 5 minutes

Assert that the rendered output for events[2] contains +5:00 (not +0:00). This is distinct from the existing TestRenderSessionDetailBothTimestampsNone test which uses a single event with no timestamp and only asserts the render does not raise.

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