feat(core): expose server API in Code Mode

[rekram1-node]

Summary

• derive the running server API document directly from OpenApi.fromApi(Api) and pass it to the Code Mode OpenAPI adapter
• expose the returned subtree under tools.opencode in the existing execute runtime
• pass the server Basic authorization header through OpenAPI.fromSpec and resolve generated requests against the server's dynamically bound address

Testing

• bun typecheck in packages/core
• bun typecheck in packages/server
• bun test test/session-runner-tool-registry.test.ts in packages/core (18 passed)
• started serve --stdio --port 0 with explicit credentials and verified authenticated GET /api/health