test: add idle-resumed and light-tier coverage for report helpers (#1162)

[microsasa]

Closes #1162

Changes

Adds three tests to close coverage gaps identified in the issue:

Gap 1 — _compute_session_totals: idle-resumed state

test_shutdown_only_no_subtraction_for_idle_resumed_session — covers the untested else branch when shutdown_only=True, has_shutdown_metrics=True, but has_active_period_stats() returns False (all active_* counters are 0, last_resume_time is None). Asserts full model_calls and user_messages are returned without subtraction.

Gap 2 — _estimate_premium_cost: light-tier 0.33× multiplier

test_light_tier_model_uses_fractional_multiplier — exercises claude-haiku-4.5 (0.33× multiplier) at 1, 3, and 10 calls, verifying the round() rounding behavior that was previously untested:

• round(0.33) = 0 → "~0"
• round(0.99) = 1 → "~1"
• round(3.3) = 3 → "~3"

Companion — render_live_sessions light-tier Est. Cost column

test_est_cost_light_tier_model — end-to-end render test that verifies a claude-haiku-4.5 session with 3 active model calls shows "~1" in the Est. Cost column.

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[Copilot]

Pull request overview

This PR adds focused regression tests for the reporting layer in copilot_usage, closing coverage gaps around idle-resumed session aggregation and light-tier premium-cost estimation/rendering.

Changes:

• Add a render_live_sessions test for a light-tier claude-haiku-4.5 session showing the expected estimated cost.
• Add direct _estimate_premium_cost assertions for 0.33× multiplier rounding behavior.
• Add _compute_session_totals coverage for idle-resumed sessions where shutdown-only totals must not subtract inactive active_* counters.