[aw][code health] Bundle of code quality cleanups in report.py and models.py

[microsasa]

Four small but genuine code-quality problems that together form a meaningful cleanup. None are caught by ruff/pyright/bandit in the current configuration.

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1. Duplicated running-time computation (report.py)

The 4-line expression to compute a session's "running" display string is copy-pasted verbatim in two functions:

# render_live_sessions, lines 147–151
running = (
    _format_elapsed_since(s.last_resume_time or s.start_time)
    if s.start_time
    else "—"
)

# _render_active_section, lines 791–795 — identical
running = (
    _format_elapsed_since(s.last_resume_time or s.start_time)
    if s.start_time
    else "—"
)

Fix: Extract a private helper _format_session_running_time(session: SessionSummary) -> str and call it from both sites.

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2. render_cost_view shadows its sessions parameter (report.py:850)

def render_cost_view(
    sessions: list[SessionSummary],
    ...
) -> None:
    console = target_console or Console()
    sessions = _filter_sessions(sessions, since, until)  # ← shadows parameter

Reassigning a parameter to a filtered subset is confusing and could silently affect readers who add code after the assignment. Every other function that filters uses a separate filtered variable.

Fix: Rename the reassignment to filtered = _filter_sessions(sessions, since, until) and update all references inside the function.

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3. Inconsistent output-token summation style (report.py)

_render_totals (lines 583–586) uses a nested for loop:

total_output = 0
for s in sessions:
    for mm in s.model_metrics.values():
        total_output += mm.usage.outputTokens

_render_historical_section (lines 741–742) does the identical computation as a comprehension:

total_output = sum(
    mm.usage.outputTokens for s in historical for mm in s.model_metrics.values()
)

A private helper _estimated_output_tokens already exists but only handles a single SessionSummary.

Fix: Standardize on the comprehension form in _render_totals. Optionally add a multi-session variant of the helper.

---

4. AssistantMessageData.toolRequests uses an unnecessarily complex default_factory (models.py:127)

toolRequests: list[dict[str, object]] = Field(
    default_factory=lambda: list[dict[str, object]]()  # ← equivalent to `list`
)

At runtime, list[dict[str, object]]() evaluates to list() — it creates a plain empty list. The generic subscript is erased. This is:

• Misleading: looks as if the factory creates a typed list at runtime, but it doesn't
• Inconsistent with all other list and dict fields in the file, which use default_factory=list / default_factory=dict

Fix:

toolRequests: list[dict[str, object]] = Field(default_factory=list)

---

Acceptance Criteria

• _format_session_running_time(session: SessionSummary) -> str helper extracted; both render_live_sessions and _render_active_section call it
• render_cost_view uses filtered (not sessions) for the filtered result; all internal references updated
• _render_totals uses the same comprehension pattern as _render_historical_section
• AssistantMessageData.toolRequests uses default_factory=list
• All existing tests pass without modification
• New unit test for _format_session_running_time: verify it returns "—" when start_time is None, returns a string from _format_elapsed_since otherwise (covering both last_resume_time present and absent)

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