fix(responses): preserve incomplete terminal state

[gruffwy]

Problem

Responses requests whose upstream completion ends with finish_reason: length are currently reported as completed and logged as successful. That loses the truncation signal for both API clients and operators.

Behavior

• Map finish_reason: length to status: incomplete.
• Include incomplete_details.reason: max_output_tokens.
• Emit response.incomplete instead of response.completed for streaming responses.
• Record request logs with an incomplete status.
• Include incomplete requests in statistics and expose them in the dashboard and request-log filter.
• Keep success-rate calculation based on fully successful requests.

Tests

• go test ./internal/gateway ./internal/accounts
• go test ./internal/store -run '^TestSummarizeRequestLogs$' -count=1
• GOFLAGS=-buildvcs=false go test ./internal/app -run 'TestImportConstraints|TestDutyBoundaries' -count=1
• (cd frontend && npm run sync)
• (cd frontend && npm run lint) — 0 errors; 17 pre-existing warnings
• git diff --check

Environment notes

The full internal/store package run still hits existing Windows-only environment failures around SQLite temporary-file cleanup and Unix permission-bit expectations; the directly affected statistics test passes.