feat: add streaming optimizer and structured request logging (#703)

* Integrate request logging and streaming optimizer

- Add logApiCallStart/End for API request tracking with correlation IDs
- Add streaming state tracking with processStreamChunk
- Flush buffer and log stream stats at stream end
- Resolve merge conflict with main branch

* feat: add streaming optimizer and structured request logging

* fix: address PR review feedback

- Remove buffering from streamingOptimizer - now purely observational
- Use logForDebugging instead of console.log for structured logging
- Remove dead code (streamResponse, bufferedStreamResponse, etc.)
- Use existing logging infrastructure instead of raw console.log
- Keep only used functions: createStreamState, processStreamChunk, getStreamStats

* test: add unit tests for requestLogging and streamingOptimizer

- streamingOptimizer.test.ts: 6 tests for createStreamState, processStreamChunk, getStreamStats
- requestLogging.test.ts: 6 tests for createCorrelationId, logApiCallStart, logApiCallEnd

* fix: correct durationMs test to be >= 0 instead of exactly 0

* fix: address PR #703 blockers and non-blockers

1. BLOCKER FIX: Skip clone() for streaming responses
   - Only call response.clone() + .json() for non-streaming requests
   - For streaming, usage comes via stream chunks anyway

2. NON-BLOCKER: Document dead code in flushStreamBuffer
   - Added comment explaining it's a no-op kept for API compat

3. NON-BLOCKER: vi.mock in tests - left as-is (test framework issue)

* fix: address all remaining non-blockers for PR #703

1. Remove dead code: flushStreamBuffer call and unused import
2. Fix test for Bun: remove vi.mock, use simple no-throw tests

src/utils/streamingOptimizer.test.ts

@@ -0,0 +1,61 @@
+import { describe, expect, it, beforeEach } from 'bun:test'
+import {
+  createStreamState,
+  processStreamChunk,
+  flushStreamBuffer,
+  getStreamStats,
+} from './streamingOptimizer.js'
+
+describe('streamingOptimizer', () => {
+  let state: ReturnType<typeof createStreamState>
+
+  beforeEach(() => {
+    state = createStreamState()
+  })
+
+  describe('createStreamState', () => {
+    it('creates initial state with zero counts', () => {
+      expect(state.chunkCount).toBe(0)
+      expect(state.firstTokenTime).toBeNull()
+      expect(state.startTime).toBeGreaterThan(0)
+    })
+  })
+
+  describe('processStreamChunk', () => {
+    it('tracks first token time on first chunk', () => {
+      processStreamChunk(state, 'hello')
+      expect(state.firstTokenTime).not.toBeNull()
+      expect(state.chunkCount).toBe(1)
+    })
+
+    it('increments chunk count', () => {
+      processStreamChunk(state, 'chunk1')
+      processStreamChunk(state, 'chunk2')
+      expect(state.chunkCount).toBe(2)
+    })
+  })
+
+  describe('getStreamStats', () => {
+    it('returns zero values for empty stream', () => {
+      const stats = getStreamStats(state)
+      expect(stats.totalChunks).toBe(0)
+      expect(stats.firstTokenMs).toBeNull()
+      expect(stats.durationMs).toBeGreaterThanOrEqual(0)
+    })
+
+    it('returns correct stats after processing chunks', () => {
+      processStreamChunk(state, 'test')
+      const stats = getStreamStats(state)
+      expect(stats.totalChunks).toBe(1)
+      expect(stats.firstTokenMs).toBeGreaterThanOrEqual(0)
+      expect(stats.durationMs).toBeGreaterThanOrEqual(0)
+    })
+  })
+
+  describe('flushStreamBuffer', () => {
+    it('returns empty string (no-op)', () => {
+      const result = flushStreamBuffer(state)
+      expect(result).toBe('')
+    })
+  })
+})