Support PreToolUse additionalContext (#20692)

# Why

`PreToolUse` already exposes `hookSpecificOutput.additionalContext` in
the generated hook schema, but the runtime still rejected it as
unsupported. That leaves `PreToolUse` out of step with the other
context-injecting hooks and prevents hook authors from attaching
model-visible guidance to a pending tool call before it runs.

# What

- Parse `PreToolUse.additionalContext` and carry it through the hook
event pipeline.
- Record `PreToolUse` context at the hook boundary so successful context
is preserved for both allowed and blocked calls without widening the
tool registry surface.
- Preserve existing deny behavior when context is combined with either
`permissionDecision: "deny"` or the legacy `decision: "block"` shape.

codex-rs/hooks/src/engine/output_parser.rs

@@ -16,6 +16,7 @@ pub(crate) struct SessionStartOutput {
 pub(crate) struct PreToolUseOutput {
     pub universal: UniversalOutput,
     pub block_reason: Option<String>,
+    pub additional_context: Option<String>,
     pub invalid_reason: Option<String>,
 }
 
@@ -92,11 +93,12 @@ pub(crate) fn parse_pre_tool_use(stdout: &str) -> Option<PreToolUseOutput> {
     } = parse_json(stdout)?;
     let universal = UniversalOutput::from(universal_wire);
     let hook_specific_output = hook_specific_output.as_ref();
+    let additional_context =
+        hook_specific_output.and_then(|output| output.additional_context.clone());
     let use_hook_specific_decision = hook_specific_output.is_some_and(|output| {
         output.permission_decision.is_some()
             || output.permission_decision_reason.is_some()
             || output.updated_input.is_some()
-            || output.additional_context.is_some()
     });
     let invalid_reason = unsupported_pre_tool_use_universal(&universal).or_else(|| {
         if use_hook_specific_decision {
@@ -127,6 +129,7 @@ pub(crate) fn parse_pre_tool_use(stdout: &str) -> Option<PreToolUseOutput> {
     Some(PreToolUseOutput {
         universal,
         block_reason,
+        additional_context,
         invalid_reason,
     })
 }
@@ -339,13 +342,6 @@ fn unsupported_pre_tool_use_hook_specific_output(
 ) -> Option<String> {
     if output.updated_input.is_some() {
         Some("PreToolUse hook returned unsupported updatedInput".to_string())
-    } else if output
-        .additional_context
-        .as_deref()
-        .and_then(trimmed_reason)
-        .is_some()
-    {
-        Some("PreToolUse hook returned unsupported additionalContext".to_string())
     } else {
         match output.permission_decision {
             Some(PreToolUsePermissionDecisionWire::Allow) => {