Introduce a new function to just send user message [Stack 3/3] (#1686)

- MCP server: add send-user-message tool to send user input to a running
Codex session
- Added an integration tests for the happy and sad paths

Changes:
•	Add tool definition and schema.
•	Expose tool in capabilities.
•	Route and handle tool requests with validation.
•	Tests for success, bad UUID, and missing session.


follow‑ups
• Listen path not implemented yet; the tool is present but marked “don’t
use yet” in code comments.
• Session run flag reset: clear running_session_id_set appropriately
after turn completion/errors.

This is the third PR in a stack.
Stack:
Final: #1686
Intermediate: #1751
First: #1750

codex-rs/mcp-server/src/mcp_protocol.rs

@@ -132,32 +132,32 @@ impl From<ToolCallResponse> for CallToolResult {
             is_error,
             result,
         } = val;
-        let (content, structured_content, is_error_out) = match result {
+        match result {
             Some(res) => match serde_json::to_value(&res) {
-                Ok(v) => {
-                    let content = vec![ContentBlock::TextContent(TextContent {
+                Ok(v) => CallToolResult {
+                    content: vec![ContentBlock::TextContent(TextContent {
                         r#type: "text".to_string(),
                         text: v.to_string(),
                         annotations: None,
-                    })];
-                    (content, Some(v), is_error)
-                }
-                Err(e) => {
-                    let content = vec![ContentBlock::TextContent(TextContent {
+                    })],
+                    is_error,
+                    structured_content: Some(v),
+                },
+                Err(e) => CallToolResult {
+                    content: vec![ContentBlock::TextContent(TextContent {
                         r#type: "text".to_string(),
                         text: format!("Failed to serialize tool result: {e}"),
                         annotations: None,
-                    })];
-                    (content, None, Some(true))
-                }
+                    })],
+                    is_error: Some(true),
+                    structured_content: None,
+                },
+            },
+            None => CallToolResult {
+                content: vec![],
+                is_error,
+                structured_content: None,
             },
-            None => (vec![], None, is_error),
-        };
-
-        CallToolResult {
-            content,
-            is_error: is_error_out,
-            structured_content,
         }
     }
 }