Add Responses API with streaming, history, and advanced features

Implements Phases 2, 3, and 6: Complete Responses API implementation

Core API (Phase 2):
- OpenAI Responses API format with structured output items
- Streaming and non-streaming support via SSE-Starlette
- Reasoning items (thinking summaries)
- Function call items (tool execution)
- Message items (assistant responses)
- Router, schemas, service, and streaming coordinator

Conversation History (Phase 3):
- Hybrid client/server approach
- Auto-generated deterministic conversation IDs
- Configurable max turns with automatic trimming
- Context window management with token counting
- Token usage statistics
- Placeholder for future vector memory integration

Advanced Features (Phase 6):
- Parameter validation with Pydantic field validators:
  - Temperature: 0.0-2.0 range enforcement
  - Reasoning effort: 6 levels (none to xhigh)
  - Max output tokens: positive integer enforcement
  - Stop sequences: up to 4, non-empty strings
- Real-time stop sequence detection during streaming
- Real-time max tokens enforcement with token counting
- Graceful error handling and OpenAI-compatible error format

Testing:
- 9 unit tests for API endpoints and streaming
- 11 unit tests for error handling
- 13 unit tests for conversation history and context
- 12 unit tests for advanced features and validation
- Total: 45 tests with comprehensive coverage

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/responses/__init__.py

@@ -0,0 +1,27 @@
+"""
+Responses API implementation.
+
+This module implements the OpenAI Responses API format:
+- /v1/responses endpoint
+- Output items (reasoning, function_call, message)
+- Streaming events
+- Multi-turn conversations
+"""
+
+from src.responses.schemas import (
+    Response,
+    ResponseRequest,
+    OutputItem,
+    MessageOutputItem,
+    ReasoningOutputItem,
+    FunctionCallOutputItem,
+)
+
+__all__ = [
+    "Response",
+    "ResponseRequest",
+    "OutputItem",
+    "MessageOutputItem",
+    "ReasoningOutputItem",
+    "FunctionCallOutputItem",
+]