Distinguish retryable thread-store errors from unknown write outcomes (#54664)

## What changed

Add `ThreadStoreError::Retryable` for transient failures known to be safe to retry and `ThreadStoreError::OutcomeUnknown` for writes that may already have committed. Recovery remains the operation owner's responsibility; neither category automatically triggers retries.

Preserve existing app-server error mappings and fatal session startup behavior, and report the new categories in shutdown diagnostics without exposing raw error payloads.

## Testing

Add coverage for conservative SQLx error classification, mutation error mappings, session initialization categories, and shutdown diagnostics that omit raw error payloads.

GitOrigin-RevId: 259a9bfaa6ff3cbfa2ee61a01a1f1e744c90f118

codex-rs/thread-store/src/error.rs

@@ -16,7 +16,11 @@ pub(crate) fn reject_paginated_history_mode(
 }
 
 /// Error type shared by thread-store implementations.
-#[derive(Debug, thiserror::Error)]
+///
+/// Implementations must explicitly distinguish known transient failures from writes whose
+/// outcome is unknown. These categories carry information only: the operation owner decides
+/// whether and how to recover, and no variant automatically triggers a retry.
+#[derive(Clone, Debug, thiserror::Error)]
 pub enum ThreadStoreError {
     /// The requested thread does not exist in this store.
     #[error("thread {thread_id} not found")]
@@ -32,7 +36,7 @@ pub enum ThreadStoreError {
         message: String,
     },
 
-    /// The operation conflicted with current store state.
+    /// The operation conflicted with current store state. This alone does not imply retryability.
     #[error("thread-store conflict: {message}")]
     Conflict {
         /// User-facing explanation of the conflict.
@@ -46,10 +50,37 @@ pub enum ThreadStoreError {
         operation: &'static str,
     },
 
+    /// A transient failure for which retrying the operation is safe.
+    ///
+    /// For a write, the implementation must know the failed attempt did not commit. Otherwise
+    /// use [`Self::OutcomeUnknown`]. Retry scheduling and limits belong to the operation owner.
+    #[error("thread-store retryable error: {message}")]
+    Retryable {
+        /// User-facing explanation of the transient failure.
+        message: String,
+    },
+
+    /// A write failed to report a definitive outcome and may already have committed.
+    ///
+    /// The operation owner must reconcile the stored state or use an idempotent replay strategy
+    /// before retrying. This must not be treated as an ordinary retryable failure.
+    #[error("thread-store write outcome unknown: {message}")]
+    OutcomeUnknown {
+        /// User-facing explanation of why the write outcome is uncertain.
+        message: String,
+    },
+
     /// Catch-all for implementation failures that do not fit a more specific category.
+    ///
+    /// This includes unclassified failures: callers must not infer retryability or whether a
+    /// write committed from this variant or its message.
     #[error("thread-store internal error: {message}")]
     Internal {
         /// User-facing explanation of the implementation failure.
         message: String,
     },
 }
+
+#[cfg(test)]
+#[path = "error_tests.rs"]
+mod tests;