external-agent-migration: avoid mixed MCP transport configs (#26435)

## Why

MCP migration could recursively merge an imported server into an
existing same-named Codex server. When one definition used stdio and the
other used HTTP, this produced an invalid mixed configuration containing
both `command` and `url`.

## What changed

- Merge MCP configuration at the server level instead of field by field.
- Preserve an existing same-named Codex MCP server unchanged.
- Report only MCP servers that would actually be added during detection.
- Add regression coverage for mixed command/HTTP source configurations.
- Use neutral fixture names and reserved `example.com` URLs.

## Test plan

- `just test -p codex-app-server repo_mcp`
  - 5 tests passed.
- `just test -p codex-external-agent-migration
mcp_migration_prefers_command_transport_for_mixed_server_config`
  - 1 test passed.

codex-rs/external-agent-migration/src/lib.rs

@@ -1451,6 +1451,49 @@ mod tests {
         );
     }
 
+    #[test]
+    fn mcp_migration_prefers_command_transport_for_mixed_server_config() {
+        let root = tempfile::TempDir::new().expect("tempdir");
+        fs::write(
+            root.path().join(".mcp.json"),
+            r#"{
+              "mcpServers": {
+                "mixedTransport": {
+                  "command": "mcp-remote-proxy",
+                  "args": [
+                    "https://example.com/mixed-transport",
+                    "--transport",
+                    "http"
+                  ],
+                  "url": "https://example.com/mixed-transport"
+                }
+              }
+            }"#,
+        )
+        .expect("write mcp");
+
+        assert_eq!(
+            build_mcp_config_from_external(
+                root.path(),
+                /*external_agent_home*/ None,
+                /*settings*/ None,
+            )
+            .unwrap(),
+            toml::from_str(
+                r#"
+[mcp_servers.mixedTransport]
+command = "mcp-remote-proxy"
+args = [
+  "https://example.com/mixed-transport",
+  "--transport",
+  "http",
+]
+"#
+            )
+            .unwrap()
+        );
+    }
+
     #[test]
     fn mcp_migration_skips_unsupported_transports() {
         let root = tempfile::TempDir::new().expect("tempdir");