[Feat] Report OpenCode setup phases and model usage in Fast turn diagnostics

[mrubens]

Why

Fast's Turn finished line lumps server spawn, session validation, and session creation into one inferenceSetupDurationMs, and says nothing about how many model requests a turn made, how big its context was, or how much inference ran after the last visible reply. On the nightly, model time to first reply is 9–21 s and the turn keeps running 10–25 s after the closeout; without these numbers there's no way to tell a slow provider from an oversized prompt or a wasted trailing request.

What

• runNonTaskSdkPrompt times each setup phase (server lease, session validate, session create, event subscribe) and passes them to onPromptStarted. Completed-message callbacks now carry the token usage OpenCode attached to the message.
• FastAgentTurnDiagnostics aggregates them, deduplicating the final prompt result against the event stream by message id, and adds to the log line and the fast_turn_settled telemetry event:
  • openCodeServerLeaseMs, openCodeSessionValidateMs, openCodeSessionCreateMs, openCodeEventSubscribeMs, openCodeSetupMs
  • modelRequestCount, completedModelRequestCount, firstModelResponseDurationMs, postReplyInferenceDurationMs
  • inputTokens, cacheReadTokens, cacheWriteTokens, outputTokens, reasoningTokens, maxContextTokens
  • systemPromptChars, environmentCount, integrationCount, integrationToolCount, activeTaskCount
• Counts and durations only; no prompt, reply, or tool content is logged.

Sample (local turn)

openCodeServerLeaseMs=2020 openCodeSessionCreateMs=44 openCodeSetupMs=2065
modelRequestCount=2 firstModelResponseDurationMs=3426 postReplyInferenceDurationMs=1648
inputTokens=10897 cacheReadTokens=10624 outputTokens=37 reasoningTokens=0 maxContextTokens=10790
systemPromptChars=29388 environmentCount=4 integrationToolCount=0

Verification

• New diagnostics test covers setup phases, request counting, token totals, dedup of the repeated final message, and the retry-attempt setup not overriding the first.
• src/server/fast-agent and provider-usage suites: 410 tests pass. Typecheck, oxlint, knip, and pre-push clean.

[roomote-community]

No code issues found. See task

• fast_turn_settled omits several newly collected diagnostics from telemetry (packages/cloud-agents/src/server/fast-agent/fast-agent-turn-diagnostics.ts:410).

_{Reviewed 13c6275}