feat(core): use experiment flags for default fetch timeouts

Replaces hardcoded fetch timeouts in `undici`'s Agent and ProxyAgent with
newly defined experiment flags `DEFAULT_REQUEST_TIMEOUT` and
`DEFAULT_TOTAL_REQUEST_TIMEOUT`. This allows for remote configuration
of request timeouts.

- Added `DEFAULT_REQUEST_TIMEOUT` and `DEFAULT_TOTAL_REQUEST_TIMEOUT` to `ExperimentFlags`.
- Updated `fetch.ts` to use `DEFAULT_REQUEST_TIMEOUT` for headers and body timeouts.
- Added unit tests to verify `setGlobalProxy` correctly applies the timeout flags.

Author: yunaseoul

packages/core/src/config/config.ts

@@ -1593,6 +1593,14 @@ export class Config implements McpContext, AgentLoopContext {
       this.setModel(DEFAULT_GEMINI_MODEL_AUTO);
     }
 
+    // Fetch admin controls
+    const experiments = await this.experimentsPromise;
+
+    const requestTimeoutMs = this.getRequestTimeoutMs();
+    if (requestTimeoutMs !== undefined) {
+      updateGlobalFetchTimeouts(requestTimeoutMs);
+    }
+
     const adminControlsEnabled =
       experiments?.flags[ExperimentFlags.ENABLE_ADMIN_CONTROLS]?.boolValue ??
       false;