Require content kinds for extension prompt fragments (#40177)

## What changed

- Require each `PromptFragment` to include a producer-owned `ContentItemKind`.
- Re-export `ContentItemKind` from `codex_extension_api` and assign stable kinds to skills, memories, examples, and test contributors.
- Limit prompt slots to developer policy and developer capabilities, and combine contributed fragments into a single developer update.

## Testing

- Update extension registry tests to verify fragment ordering with the new content-kind metadata.

GitOrigin-RevId: ef8624a9b4ae9dce025bca9c0954d91dd8075e85

codex-rs/ext/extension-api/src/lib.rs

@@ -17,6 +17,7 @@ pub use capabilities::NoopResponseItemInjector;
 pub use capabilities::ResponseItemInjectionFuture;
 pub use capabilities::ResponseItemInjector;
 pub use codex_context_fragments::ContextualUserFragment;
+pub use codex_protocol::models::ContentItemKind;
 pub use codex_protocol::models::ResponseItem;
 pub use codex_protocol::security_risk::SecurityRiskScore;
 pub use codex_tools::ConversationHistory;