Support openai/elicitation form requests (#41447)

## What changed

- Advertise `openai/elicitation` form support when the client declares an
  object-valued `form` capability, without deriving it from the legacy
  `openai/form` capability.
- Handle `openai/elicitation/create` requests in `form` mode and forward their
  metadata and opaque schema through app-server as `openaiForm`, preserving
  `x-openai-*` annotations.
- Keep legacy form handling independent and have the TUI automatically decline
  the new form requests it cannot render.

## Testing

- Add an app-server round-trip test for capability negotiation, request
  forwarding, and accepted responses.
- Extend MCP capability filtering coverage for `openai/elicitation.form`.

GitOrigin-RevId: 88f39257bffafe4ee98a9c910e38507843cd7eeb

codex-rs/core/src/session/mcp.rs

@@ -942,7 +942,9 @@ fn guardian_elicitation_review_request(
                 "guardian MCP elicitation review does not support this elicitation mode",
             );
         }
-        Elicitation::OpenAiForm { .. } => return GuardianElicitationReview::NotRequested,
+        Elicitation::OpenAiForm { .. } | Elicitation::OpenAiElicitationForm { .. } => {
+            return GuardianElicitationReview::NotRequested;
+        }
     };
 
     let Some(meta) = meta.as_ref().map(|meta| &meta.0.0) else {