test: add integration test for MCP server (#1633)

This PR introduces a single integration test for `cargo mcp`, though it
also introduces a number of reusable components so that it should be
easier to introduce more integration tests going forward.

The new test is introduced in `codex-rs/mcp-server/tests/elicitation.rs`
and the reusable pieces are in `codex-rs/mcp-server/tests/common`.

The test itself verifies new functionality around elicitations
introduced in https://github.com/openai/codex/pull/1623 (and the fix
introduced in https://github.com/openai/codex/pull/1629) by doing the
following:

- starts a mock model provider with canned responses for
`/v1/chat/completions`
- starts the MCP server with a `config.toml` to use that model provider
(and `approval_policy = "untrusted"`)
- sends the `codex` tool call which causes the mock model provider to
request a shell call for `git init`
- the MCP server sends an elicitation to the client to approve the
request
- the client replies to the elicitation with `"approved"`
- the MCP server runs the command and re-samples the model, getting a
`"finish_reason": "stop"`
- in turn, the MCP server sends the final response to the original
`codex` tool call
- verifies that `git init` ran as expected

To test:

```
cargo test shell_command_approval_triggers_elicitation
```

In writing this test, I discovered that `ExecApprovalResponse` does not
conform to `ElicitResult`, so I added a TODO to fix that, since I think
that should be updated in a separate PR. As it stands, this PR does not
update any business logic, though it does make a number of members of
the `mcp-server` crate `pub` so they can be used in the test.

One additional learning from this PR is that
`std::process::Command::cargo_bin()` from the `assert_cmd` trait is only
available for `std::process::Command`, but we really want to use
`tokio::process::Command` so that everything is async and we can
leverage utilities like `tokio::time::timeout()`. The trick I came up
with was to use `cargo_bin()` to locate the program, and then to use
`std::process::Command::get_program()` when constructing the
`tokio::process::Command`.

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

@@ -100,7 +100,10 @@ pub async fn run_codex_tool_session(
                     }) => {
                         let escaped_command = shlex::try_join(command.iter().map(|s| s.as_str()))
                             .unwrap_or_else(|_| command.join(" "));
-                        let message = format!("Allow Codex to run `{escaped_command}` in {cwd:?}?");
+                        let message = format!(
+                            "Allow Codex to run `{escaped_command}` in `{cwd}`?",
+                            cwd = cwd.to_string_lossy()
+                        );
 
                         let params = ExecApprovalElicitRequestParams {
                             message,
@@ -276,7 +279,12 @@ async fn on_exec_approval_response(
     }
 }
 
-#[derive(Debug, Deserialize)]
+// TODO(mbolin): ExecApprovalResponse does not conform to ElicitResult. See:
+// - https://github.com/modelcontextprotocol/modelcontextprotocol/blob/f962dc1780fa5eed7fb7c8a0232f1fc83ef220cd/schema/2025-06-18/schema.json#L617-L636
+// - https://modelcontextprotocol.io/specification/draft/client/elicitation#protocol-messages
+// It should have "action" and "content" fields.
+
+#[derive(Debug, Serialize, Deserialize)]
 pub struct ExecApprovalResponse {
     pub decision: ReviewDecision,
 }
@@ -284,19 +292,19 @@ pub struct ExecApprovalResponse {
 /// Conforms to [`mcp_types::ElicitRequestParams`] so that it can be used as the
 /// `params` field of an [`mcp_types::ElicitRequest`].
 #[derive(Debug, Serialize)]
-struct ExecApprovalElicitRequestParams {
+pub struct ExecApprovalElicitRequestParams {
     // These fields are required so that `params`
     // conforms to ElicitRequestParams.
-    message: String,
+    pub message: String,
 
     #[serde(rename = "requestedSchema")]
-    requested_schema: ElicitRequestParamsRequestedSchema,
+    pub requested_schema: ElicitRequestParamsRequestedSchema,
 
     // These are additional fields the client can use to
     // correlate the request with the codex tool call.
-    codex_elicitation: String,
-    codex_mcp_tool_call_id: String,
-    codex_event_id: String,
-    codex_command: Vec<String>,
-    codex_cwd: PathBuf,
+    pub codex_elicitation: String,
+    pub codex_mcp_tool_call_id: String,
+    pub codex_event_id: String,
+    pub codex_command: Vec<String>,
+    pub codex_cwd: PathBuf,
 }