Treat model-not-found provider errors as terminal failures (#56445)

## Why

Provider errors with code `model_not_found` should end the turn immediately and preserve the provider's message instead of triggering retries.

## What changed

- Recognize `model_not_found` in HTTP 400, 403, and 404 responses and in streamed `error` and `response.failed` events.
- Preserve the provider message and any available HTTP status, and expose `modelNotFound` through the app-server protocol and generated schemas and SDK types.

## Testing

Add coverage for HTTP classification, statusless stream errors, and terminal failures without retries across HTTP, SSE, and WebSocket responses. Verify that `thread/read` preserves the error reported by the live notification.

GitOrigin-RevId: 88f824ac03c8c9669915b1e40c67b76c08326fbe

codex-rs/app-server-protocol/src/protocol/v2/shared.rs

@@ -80,6 +80,7 @@ pub enum CodexErrorInfo {
     RateLimitExceeded,
     FlexUnavailable,
     ServerOverloaded,
+    ModelNotFound,
     CyberPolicy,
     MisalignmentPolicyViolation,
     TooManyDenials,
@@ -155,6 +156,7 @@ impl From<CoreCodexErrorInfo> for CodexErrorInfo {
             CoreCodexErrorInfo::RateLimitExceeded => CodexErrorInfo::RateLimitExceeded,
             CoreCodexErrorInfo::FlexUnavailable => CodexErrorInfo::FlexUnavailable,
             CoreCodexErrorInfo::ServerOverloaded => CodexErrorInfo::ServerOverloaded,
+            CoreCodexErrorInfo::ModelNotFound => CodexErrorInfo::ModelNotFound,
             CoreCodexErrorInfo::CyberPolicy => CodexErrorInfo::CyberPolicy,
             CoreCodexErrorInfo::BioPolicy => CodexErrorInfo::Other,
             CoreCodexErrorInfo::MisalignmentPolicyViolation => {