Reduce weather-backed agent answer completion latency

Goal: keep simple weather-backed prompts feeling fast end to end, not just fast to first event. Scope: - Focus on the existing web /agent weather path and shared synthesis/streaming code as needed. - Preserve public contracts for /agent, MCP, A2A, REST, webhook, and env names. - Do not add new product surface area. Acceptance criteria: - A guest prompt such as "Weather in New York today" still emits early progress/tool status promptly. - The final concise weather answer completes quickly instead of trickling a one-sentence response over many seconds after the weather tool succeeds. - The answer remains grounded in provider data, includes the request-date context when relevant, and keeps unavailable/stale weather data explicit. - Add or update CI-verifiable coverage for the latency/streaming behavior without relying on live provider calls.