feat(adk): add Streamable HTTP middleware and restructure SSE middleware

- Add StreamableMiddleware for MCP 2025-03-26 Streamable HTTP transport
  - Single POST /mcp endpoint with Mcp-Session-Id header
  - Supports JSON response and SSE streaming upgrade
  - Supports stateless mode and notification handling
- Rename middleware.go -> sse_middleware.go for clarity
- Rename middleware_test.go -> sse_middleware_test.go
- Delete redundant mcp_middleware.go (merged into sse_middleware.go)
- Rewrite e2e_test.go to cover both SSE and Streamable HTTP transports
  - Fix SSE client to use proper MCP protocol (GET /sse + POST /messages)
  - Add Streamable HTTP e2e tests (basic, session continuation, streaming)
  - HA scenarios (migration, failover) remain transport-agnostic

Author: jizhuozhi