feat(m3a): spawn bridge — bubble child session costs to parent

[kenotron-ms]

What

Adds spawn-boundary cost bridging to the app-cli session spawner as part of the M3 cost management milestone.

Background

M2 providers register a session.cost contributor in each session that tracks cumulative cost_usd across all LLM calls. Without bridging, child session costs stay isolated — the parent sees only its own direct LLM calls when collecting contributions.

How it works

Two helpers are added to session_spawner.py:

_sum_cost_usd(contributions) — Reduces the list returned by collect_contributions("session.cost") to a single Decimal total, or None when no cost data is present (e.g. self-hosted models). None propagates correctly rather than masking as zero.

_bridge_child_cost(child_coordinator, parent_coordinator, child_session_id) — Called after child_session.execute() returns and before child_session.cleanup() so the child coordinator is still alive. Collects the child's cost contributions, sums them, and registers the frozen total as a new contributor on the parent coordinator's session.cost channel, keyed as "delegate:{child_session_id}".

Coverage

Both spawn sites are patched:

• spawn_sub_session() — new child sessions
• resume_sub_session() — resumed sessions

After these changes, a parent session's collect_contributions("session.cost") includes costs from every child spawned in that session, enabling accurate session-total cost display in M3's turn footer.

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Tracks: microsoft-amplifier/amplifier-support#225