feat: replace custom mcp-types crate with equivalents from rmcp (#10349)

We started working with MCP in Codex before
https://crates.io/crates/rmcp was mature, so we had our own crate for
MCP types that was generated from the MCP schema:


8b95d3e082/codex-rs/mcp-types/README.md

Now that `rmcp` is more mature, it makes more sense to use their MCP
types in Rust, as they handle details (like the `_meta` field) that our
custom version ignored. Though one advantage that our custom types had
is that our generated types implemented `JsonSchema` and `ts_rs::TS`,
whereas the types in `rmcp` do not. As such, part of the work of this PR
is leveraging the adapters between `rmcp` types and the serializable
types that are API for us (app server and MCP) introduced in #10356.

Note this PR results in a number of changes to
`codex-rs/app-server-protocol/schema`, which merit special attention
during review. We must ensure that these changes are still
backwards-compatible, which is possible because we have:

```diff
- export type CallToolResult = { content: Array<ContentBlock>, isError?: boolean, structuredContent?: JsonValue, };
+ export type CallToolResult = { content: Array<JsonValue>, structuredContent?: JsonValue, isError?: boolean, _meta?: JsonValue, };
```

so `ContentBlock` has been replaced with the more general `JsonValue`.
Note that `ContentBlock` was defined as:

```typescript
export type ContentBlock = TextContent | ImageContent | AudioContent | ResourceLink | EmbeddedResource;
```

so the deletion of those individual variants should not be a cause of
great concern.

Similarly, we have the following change in
`codex-rs/app-server-protocol/schema/typescript/Tool.ts`:

```
- export type Tool = { annotations?: ToolAnnotations, description?: string, inputSchema: ToolInputSchema, name: string, outputSchema?: ToolOutputSchema, title?: string, };
+ export type Tool = { name: string, title?: string, description?: string, inputSchema: JsonValue, outputSchema?: JsonValue, annotations?: JsonValue, icons?: Array<JsonValue>, _meta?: JsonValue, };
```

so:

- `annotations?: ToolAnnotations` ➡️ `JsonValue`
- `inputSchema: ToolInputSchema` ➡️ `JsonValue`
- `outputSchema?: ToolOutputSchema` ➡️ `JsonValue`

and two new fields: `icons?: Array<JsonValue>, _meta?: JsonValue`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/10349).
* #10357
* __->__ #10349
* #10356

codex-rs/core/tests/suite/rmcp_client.rs

@@ -26,7 +26,6 @@ use core_test_support::skip_if_no_network;
 use core_test_support::stdio_server_bin;
 use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event;
-use mcp_types::ContentBlock;
 use serde_json::Value;
 use serde_json::json;
 use serial_test::serial;
@@ -307,14 +306,10 @@ async fn stdio_image_responses_round_trip() -> anyhow::Result<()> {
     let base64_only = OPENAI_PNG
         .strip_prefix("data:image/png;base64,")
         .expect("data url prefix");
-    match &result.content[0] {
-        ContentBlock::ImageContent(img) => {
-            assert_eq!(img.mime_type, "image/png");
-            assert_eq!(img.r#type, "image");
-            assert_eq!(img.data, base64_only);
-        }
-        other => panic!("expected image content, got {other:?}"),
-    }
+    let entry = result.content[0].as_object().expect("content object");
+    assert_eq!(entry.get("type"), Some(&json!("image")));
+    assert_eq!(entry.get("mimeType"), Some(&json!("image/png")));
+    assert_eq!(entry.get("data"), Some(&json!(base64_only)));
 
     wait_for_event(&fixture.codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;