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:


https://github.com/openai/codex/blob/8b95d3e082376f4cb23e92641705a22afb28a9da/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/exec/src/event_processor_with_human_output.rs

@@ -375,7 +375,8 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 ts_msg!(self, "{}", title.style(title_style));
 
                 if let Ok(res) = result {
-                    let val: serde_json::Value = res.into();
+                    let val = serde_json::to_value(res)
+                        .unwrap_or_else(|_| serde_json::Value::String("<result>".to_string()));
                     let pretty =
                         serde_json::to_string_pretty(&val).unwrap_or_else(|_| val.to_string());
 

codex-rs/exec/src/exec_events.rs

@@ -1,5 +1,4 @@
 use codex_protocol::models::WebSearchAction;
-use mcp_types::ContentBlock as McpContentBlock;
 use serde::Deserialize;
 use serde::Serialize;
 use serde_json::Value as JsonValue;
@@ -256,7 +255,14 @@ pub struct CollabToolCallItem {
 /// Result payload produced by an MCP tool invocation.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, TS)]
 pub struct McpToolCallItemResult {
-    pub content: Vec<McpContentBlock>,
+    // NOTE: `rmcp::model::Content` (and its `RawContent` variants) would be a
+    // more precise Rust representation of MCP content blocks. We intentionally
+    // use `serde_json::Value` here because this crate exports JSON schema + TS
+    // types (`schemars`/`ts-rs`), and the rmcp model types aren't set up to be
+    // schema/TS friendly (and would introduce heavier coupling to rmcp's Rust
+    // representations). Using `JsonValue` keeps the payload wire-shaped and
+    // easy to export.
+    pub content: Vec<JsonValue>,
     pub structured_content: Option<JsonValue>,
 }