Add headers field to McpTool spec

[Copilot]

MCP tools had no way to specify custom HTTP headers in declarative YAML agent definitions, blocking use cases like per-request auth tokens or routing headers.

Changes

• agentschema-emitter/lib/model/tools/mcp.tsp — Adds headers?: Record<string> to McpTool, allowing arbitrary string key-value HTTP headers
• agentschema-emitter/src/ast.ts — Extends the Record<unknown> dictionary detection to also handle Record<string>, preventing the emitter from generating an invalid standalone class for it
• Generated runtimes — All four language runtimes updated; headers surfaces as IDictionary<string, object>? (C#), Optional[dict[str, Any]] (Python), Record<string, unknown> | undefined (TypeScript), map[string]interface{} (Go)
• JSON Schema — New RecordString.yaml schema (string-valued record) referenced from McpTool.yaml

Usage

tools:
  - kind: mcp
    serverName: product-mcp
    connection:
      kind: anonymous
      endpoint: =Env.MCP_URL
    headers:
      Authorization: =Env.MCP_ACCESS_TOKEN
      X-Tenant-Id: =Env.TENANT_ID
    approvalMode: never