[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/src/tools/events.rs

@@ -116,7 +116,6 @@ pub(crate) enum ToolEmitter {
         cwd: AbsolutePathBuf,
         source: ExecCommandSource,
         parsed_cmd: Vec<ParsedCommand>,
-        freeform: bool,
     },
     ApplyPatch {
         changes: HashMap<PathBuf, FileChange>,
@@ -132,19 +131,13 @@ pub(crate) enum ToolEmitter {
 }
 
 impl ToolEmitter {
-    pub fn shell(
-        command: Vec<String>,
-        cwd: AbsolutePathBuf,
-        source: ExecCommandSource,
-        freeform: bool,
-    ) -> Self {
+    pub fn shell(command: Vec<String>, cwd: AbsolutePathBuf, source: ExecCommandSource) -> Self {
         let parsed_cmd = parse_command(&command);
         Self::Shell {
             command,
             cwd,
             source,
             parsed_cmd,
-            freeform,
         }
     }
 
@@ -328,12 +321,7 @@ impl ToolEmitter {
         output: &ExecToolCallOutput,
         ctx: ToolEventCtx<'_>,
     ) -> String {
-        match self {
-            Self::Shell { freeform: true, .. } => {
-                super::format_exec_output_for_model_freeform(output, ctx.turn.truncation_policy)
-            }
-            _ => super::format_exec_output_for_model_structured(output, ctx.turn.truncation_policy),
-        }
+        super::format_exec_output_for_model(output, ctx.turn.truncation_policy)
     }
 
     pub async fn finish(

codex-rs/core/src/tools/handlers/shell.rs

@@ -52,7 +52,6 @@ struct RunExecLikeArgs {
     turn: Arc<TurnContext>,
     tracker: crate::tools::context::SharedTurnDiffTracker,
     call_id: String,
-    freeform: bool,
     shell_runtime_backend: ShellRuntimeBackend,
 }
 
@@ -68,7 +67,6 @@ async fn run_exec_like(args: RunExecLikeArgs) -> Result<FunctionToolOutput, Func
         turn,
         tracker,
         call_id,
-        freeform,
         shell_runtime_backend,
     } = args;
 
@@ -162,12 +160,7 @@ async fn run_exec_like(args: RunExecLikeArgs) -> Result<FunctionToolOutput, Func
     }
 
     let source = ExecCommandSource::Agent;
-    let emitter = ToolEmitter::shell(
-        exec_params.command.clone(),
-        exec_params.cwd.clone(),
-        source,
-        freeform,
-    );
+    let emitter = ToolEmitter::shell(exec_params.command.clone(), exec_params.cwd.clone(), source);
     let event_ctx = ToolEventCtx::new(
         session.as_ref(),
         turn.as_ref(),

codex-rs/core/src/tools/handlers/shell/shell_command.rs

@@ -197,7 +197,6 @@ impl ToolExecutor<ToolInvocation> for ShellCommandHandler {
             turn,
             tracker,
             call_id,
-            freeform: true,
             shell_runtime_backend: self.shell_runtime_backend(),
         })
         .await

codex-rs/core/src/tools/mod.rs

@@ -23,7 +23,6 @@ use codex_utils_output_truncation::TruncationPolicy;
 use codex_utils_output_truncation::formatted_truncate_text;
 use codex_utils_output_truncation::truncate_text;
 pub use router::ToolRouter;
-use serde::Serialize;
 
 // Telemetry preview limits: keep log events smaller than model budgets.
 pub(crate) const TELEMETRY_PREVIEW_MAX_BYTES: usize = 2 * 1024; // 2 KiB
@@ -60,46 +59,7 @@ pub(crate) fn tool_user_shell_type(
 
 /// Format the combined exec output for sending back to the model.
 /// Includes exit code and duration metadata; truncates large bodies safely.
-pub fn format_exec_output_for_model_structured(
-    exec_output: &ExecToolCallOutput,
-    truncation_policy: TruncationPolicy,
-) -> String {
-    let ExecToolCallOutput {
-        exit_code,
-        duration,
-        ..
-    } = exec_output;
-
-    #[derive(Serialize)]
-    struct ExecMetadata {
-        exit_code: i32,
-        duration_seconds: f32,
-    }
-
-    #[derive(Serialize)]
-    struct ExecOutput<'a> {
-        output: &'a str,
-        metadata: ExecMetadata,
-    }
-
-    // round to 1 decimal place
-    let duration_seconds = ((duration.as_secs_f32()) * 10.0).round() / 10.0;
-
-    let formatted_output = format_exec_output_str(exec_output, truncation_policy);
-
-    let payload = ExecOutput {
-        output: &formatted_output,
-        metadata: ExecMetadata {
-            exit_code: *exit_code,
-            duration_seconds,
-        },
-    };
-
-    #[expect(clippy::expect_used)]
-    serde_json::to_string(&payload).expect("serialize ExecOutput")
-}
-
-pub fn format_exec_output_for_model_freeform(
+pub fn format_exec_output_for_model(
     exec_output: &ExecToolCallOutput,
     truncation_policy: TruncationPolicy,
 ) -> String {