[codex] Remove legacy shell output formatting paths (#22706)

## Why

The client and tool pipeline still carried compatibility code for legacy
structured shell output. Current shell and apply_patch responses are
already plain text for model consumption, so keeping a
JSON-serialization path plus shell-item rewrite logic makes the request
formatter and tests preserve a format we do not need anymore.

## What Changed

- Removed the client-side shell output rewrite from
`core/src/client_common.rs`.
- Removed the structured exec-output formatter and the shell `freeform`
switch so tool emitters use one model-facing formatter.
- Collapsed apply_patch/shell serialization tests around the remaining
plain-text output expectations and removed duplicate one-variant
parameterized cases.
- Kept the `ApplyPatchModelOutput::ShellCommandViaHeredoc` compatibility
input shape, but no longer treats it as a separate output-format mode.

## Validation

- `cargo test -p codex-core client_common`
- `cargo test -p codex-core shell_serialization`
- `cargo test -p codex-core apply_patch_cli`
- `just fix -p codex-core`

## Documentation

No external Codex documentation update is needed.

codex-rs/core/tests/common/responses.rs

@@ -35,8 +35,6 @@ use wiremock::http::HeaderValue;
 use wiremock::matchers::method;
 use wiremock::matchers::path_regex;
 
-use crate::test_codex::ApplyPatchModelOutput;
-
 #[derive(Debug, Clone)]
 pub struct ResponseMock {
     requests: Arc<Mutex<Vec<ResponsesRequest>>>,
@@ -883,19 +881,6 @@ pub fn ev_local_shell_call(call_id: &str, status: &str, command: Vec<&str>) -> V
     })
 }
 
-pub fn ev_apply_patch_call(
-    call_id: &str,
-    patch: &str,
-    output_type: ApplyPatchModelOutput,
-) -> Value {
-    match output_type {
-        ApplyPatchModelOutput::Freeform => ev_apply_patch_custom_tool_call(call_id, patch),
-        ApplyPatchModelOutput::ShellCommandViaHeredoc => {
-            ev_apply_patch_shell_command_call_via_heredoc(call_id, patch)
-        }
-    }
-}
-
 /// Convenience: SSE event for an `apply_patch` custom tool call with raw patch
 /// text. This mirrors the payload produced by the Responses API when the model
 /// invokes `apply_patch` directly.

codex-rs/core/tests/common/test_codex.rs

@@ -184,20 +184,12 @@ fn docker_command_capture_stdout<const N: usize>(args: [&str; N]) -> Result<Stri
     String::from_utf8(output.stdout).context("docker stdout must be utf-8")
 }
 
-/// A collection of different ways the model can output an apply_patch call
+/// Non-default apply_patch model output shapes used by compatibility tests.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum ApplyPatchModelOutput {
-    Freeform,
     ShellCommandViaHeredoc,
 }
 
-/// A collection of different ways the model can output an apply_patch call
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
-pub enum ShellModelOutput {
-    ShellCommand,
-    // UnifiedExec has its own set of tests
-}
-
 /// Returns the permission fields required by `Op::UserTurn` for tests that
 /// construct the op directly.
 pub fn turn_permission_fields(
@@ -977,21 +969,8 @@ impl TestCodexHarness {
         custom_tool_call_output_text(&bodies, call_id)
     }
 
-    pub async fn apply_patch_output(
-        &self,
-        call_id: &str,
-        output_type: ApplyPatchModelOutput,
-    ) -> String {
-        // Box the awaited output helpers so callers do not inline request
-        // capture and response parsing into their own async state.
-        match output_type {
-            ApplyPatchModelOutput::Freeform => {
-                Box::pin(self.custom_tool_call_output(call_id)).await
-            }
-            ApplyPatchModelOutput::ShellCommandViaHeredoc => {
-                Box::pin(self.function_call_stdout(call_id)).await
-            }
-        }
+    pub async fn apply_patch_output(&self, call_id: &str) -> String {
+        self.custom_tool_call_output(call_id).await
     }
 }