Allow guardian bare allow output (#18797)

## Summary

Allow guardian to skip other fields and output only
`{"outcome":"allow"}` when the command is low risk.
This change lets guardian reviews use a non-strict text format while
keeping the JSON schema itself as plain user-visible schema data, so
transport strictness is carried out-of-band instead of through a schema
marker key.

## What changed

- Add an explicit `output_schema_strict` flag to model prompts and pass
it into `codex-api` text formatting.
- Set guardian reviewer prompts to non-strict schema validation while
preserving strict-by-default behavior for normal callers.
- Update the guardian output contract so definitely-low-risk decisions
may return only `{"outcome":"allow"}`.
- Treat bare allow responses as low-risk approvals in the guardian
parser.
- Add tests and snapshots covering the non-strict guardian request and
optional guardian output fields.

## Verification

- `cargo test -p codex-core guardian::tests::guardian`
- `cargo test -p codex-core guardian::tests::`
- `cargo test -p codex-core client_common::tests::`
- `cargo test -p codex-protocol
user_input_serialization_includes_final_output_json_schema`
- `cargo test -p codex-api`
- `git diff --check`

Note: `cargo test -p codex-core` was also attempted, but this desktop
environment injects ambient config/proxy state that causes unrelated
config/session tests expecting pristine defaults to fail.

---------

Co-authored-by: Dylan Hurd <dylan.hurd@openai.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/src/common.rs

@@ -262,6 +262,7 @@ pub enum ResponsesWsRequest {
 pub fn create_text_param_for_request(
     verbosity: Option<VerbosityConfig>,
     output_schema: &Option<Value>,
+    output_schema_strict: bool,
 ) -> Option<TextControls> {
     if verbosity.is_none() && output_schema.is_none() {
         return None;
@@ -271,7 +272,7 @@ pub fn create_text_param_for_request(
         verbosity: verbosity.map(std::convert::Into::into),
         format: output_schema.as_ref().map(|schema| TextFormat {
             r#type: TextFormatType::JsonSchema,
-            strict: true,
+            strict: output_schema_strict,
             schema: schema.clone(),
             name: "codex_output_schema".to_string(),
         }),