[BUG]: Avoid flattening multiple model metering events into one

[tanujnay112]

Summary

• stream the existing merged per-model usage totals from /api/agent immediately before the terminal done event
• include input, output, cache-read, and cache-write token counts in each usage event
• exercise aggregation of repeated same-model records while keeping models separate

Context

• [BUG](foundation-api): Parse search usage records #7524, now on main, parses and preserves subagent_search usage_records and passes them into the existing per-model MeterEvent fanout
• this PR is the client-facing SSE follow-up; it does not change Orb/MeterEvent submission

Testing

• cargo test -p foundation-api agent::tests --tests
• cargo test -p foundation-api subagent_search --tests

[github-actions]

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
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• If appropriate, are there adequate unit tests?
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