feat(mcp): Add StreamableHTTPServerTransport support

[hggzm]

Summary

Adds support for the new MCP Streamable HTTP transport alongside existing SSE and stdio transports.

Changes

• Added McpStreamableHTTPTransportOptions type
• Implemented onInitStreamableHTTP() handler with session management
• Added POST/GET/DELETE handlers for full HTTP transport lifecycle
• Updated transport union type and switch statements

Features

• Stateful session management with auto-cleanup
• Compatible with OpenAI Agents SDK remote MCP connections
• Backwards compatible (SSE remains default)

Fixes #2470

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