Feat: appServer.requirementList for requirement.toml (#8800)

### Summary
We are exposing requirements via `requirement/list` method from
app-server so that we can conditionally disable the agent mode dropdown
selection in VSCE and correctly setting the default value.

### Sample output
#### `etc/codex/requirements.toml`
<img width="497" height="49" alt="Screenshot 2026-01-06 at 11 32 06 PM"
src="https://github.com/user-attachments/assets/fbd9402e-515f-4b9e-a158-2abb23e866a0"
/>

#### App server response
<img width="1107" height="79" alt="Screenshot 2026-01-06 at 11 30 18 PM"
src="https://github.com/user-attachments/assets/c0d669cd-54ef-4789-a26c-adb2c41950af"
/>

codex-rs/app-server/src/codex_message_processor.rs

@@ -510,6 +510,9 @@ impl CodexMessageProcessor {
             | ClientRequest::ConfigBatchWrite { .. } => {
                 warn!("Config request reached CodexMessageProcessor unexpectedly");
             }
+            ClientRequest::ConfigRequirementsRead { .. } => {
+                warn!("ConfigRequirementsRead request reached CodexMessageProcessor unexpectedly");
+            }
             ClientRequest::GetAccountRateLimits {
                 request_id,
                 params: _,