feat(analytics): generate an installation_id and pass it in responsesapi client_metadata (#16912)

## Summary

This adds a stable Codex installation ID and includes it on Responses
API requests via `x-codex-installation-id` passed in via the
`client_metadata` field for analytics/debugging.

The main pieces are:
- persist a UUID in `$CODEX_HOME/installation_id`
- thread the installation ID into `ModelClient`
- send it in `client_metadata` on Responses requests so it works
consistently across HTTP and WebSocket transports

codex-rs/core/src/client_common_tests.rs

@@ -29,6 +29,7 @@ fn serializes_text_verbosity_when_set() {
             verbosity: Some(OpenAiVerbosity::Low),
             format: None,
         }),
+        client_metadata: None,
     };
 
     let v = serde_json::to_value(&req).expect("json");
@@ -69,6 +70,7 @@ fn serializes_text_schema_with_strict_format() {
         prompt_cache_key: None,
         service_tier: None,
         text: Some(text_controls),
+        client_metadata: None,
     };
 
     let v = serde_json::to_value(&req).expect("json");
@@ -106,6 +108,7 @@ fn omits_text_when_not_set() {
         prompt_cache_key: None,
         service_tier: None,
         text: None,
+        client_metadata: None,
     };
 
     let v = serde_json::to_value(&req).expect("json");
@@ -128,6 +131,7 @@ fn serializes_flex_service_tier_when_set() {
         prompt_cache_key: None,
         service_tier: Some(ServiceTier::Flex.to_string()),
         text: None,
+        client_metadata: None,
     };
 
     let v = serde_json::to_value(&req).expect("json");