feat(session): add per-session MCP server support for REST API

[HyeongDoYun]

Summary

Adds mcpServers parameter to POST /session endpoint, enabling per-session MCP server configuration via REST API.

Changes

• Add mcpServers field to Session.create.schema
• Pass mcpServers to Session.createNext
• Register MCP servers via MCP.add() during session creation

Motivation

ACP (Agent Client Protocol) already supports per-session MCP via newSession(). This PR brings the same capability to the REST API, allowing external integrations to dynamically configure MCP servers per session.

Usage

POST /session
{
  "title": "My Session",
  "mcpServers": {
    "my-remote-mcp": {
      "type": "remote",
      "url": "https://mcp.example.com/sse"
    },
    "my-local-mcp": {
      "type": "local",
      "command": ["npx", "my-mcp-server"]
    }
  }
}

Testing

• All existing tests pass (544 pass, 0 fail)

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[HyeongDoYun]

Hi! This PR adds REST API parity with ACP for per-session MCP configuration.

Currently newSession() in ACP accepts mcpServers, but POST /session doesn't — this creates inconsistency for integrations that need dynamic MCP configuration via REST.

If this requires design review first, happy to close this and open an issue instead. Let me know!

[HyeongDoYun]

Related issue: #7004