## Summary
Adds test coverage for the three compaction-decision functions in chatd
that had zero tests: `latestPromptUsage`, `shouldCompactPromptUsage`,
and `contextTokensFromUsage`.
AIGOV-585 hypothesized that chatd's token counting logic was incorrect —
that it compared a cumulative sum of prompt tokens across all
agentic-loop steps against the context window. The tests disprove this:
`latestPromptUsage` returns the last persisted assistant message's
usage, not a sum. The actual bug was in the aibridge streaming
interceptor, which summed usage across SSE chunks and persisted inflated
values (fixed in `ad100452d4`).
## What's tested
- `TestLatestPromptUsage` — pins that the compaction path reads the last
step's usage (5,400), not a cumulative sum across steps (15,600). If
someone wires `TotalUsage` into the compaction path as the issue
suggested, this fails.
- `TestShouldCompactPromptUsage` — covers the threshold decision with
the inflated value from the issue (417,012 → compacts), the correct
value (6,000 → doesn't compact), cache token counting, and both disable
guards (threshold=100, contextLimit=0).
<details>
<summary>Plan / investigation notes</summary>
- Traced the full flow: `chatloop.go:993` sets `result.usage =
part.Usage` from the per-step `StreamPartTypeFinish` event, not the
accumulated `TotalUsage` from `agent.go:544`. chatd never calls
fantasy's `Agent` interface.
- The `TotalUsage` accumulation in `agent.go:544` is only used for cost
attribution, not context occupancy.
- Commit `ad100452d4` fixed the real bug in
`aibridge/intercept/chatcompletions/streaming.go` (cross-chunk usage
summation for vLLM-style backends).
- Tests reuse existing `dbMessage` and `withUsage` helpers from
`message_conversion_test.go` (same package).
</details>
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