fix(storage): classify blob_exists too — it is the migration's first probe

The previous commit fixed get / get-range / stat but left `blob_exists`
returning `internal_error` on S3 and Azure, which undoes the point of
the exercise: `blob_exists` is the FIRST call `backend_migration` makes
against the source for every blob.

    match self.source.blob_exists(hash).await {   // migration, per blob

An unclassified error there is permanent, so a refused connection during
a migration takes the permanent branch — record a finding and move on —
which is the skip-and-advance behaviour the pause was added to prevent.
The classification has to hold at the probe, not only at the read that
follows it.

Both now classify before deciding: only a genuine 404 / `is_not_found`
answers "absent", everything else keeps its transient class. On S3 that
means classifying the `SdkError` by reference first, since
`into_service_error()` consumes it.

Local was already routed through `local_io_error` at its stat site.

Audited the rest of the S3 surface: initialize, put ×3, get, get-range,
delete, stat, list and exists all classify. The two remaining
`internal_error`s in `put_blob` read a *local* source file, so there is
no network class to preserve.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/infrastructure/services/azure_blob_backend.rs

@@ -393,9 +393,13 @@ impl BlobStorageBackend for AzureBlobBackend {
                     if status == Some(azure_core::StatusCode::NotFound) {
                         Ok(false)
                     } else {
-                        Err(DomainError::internal_error(
-                            "Azure",
-                            format!("Failed to check blob {hash}: {e}"),
+                        // Only the 404 means "absent"; everything else keeps
+                        // its transient/permanent class so the migration's
+                        // source probe can pause on an outage instead of
+                        // recording a permanent finding.
+                        Err(azure_domain_error(
+                            format!("Failed to check blob {hash}"),
+                            &e,
                         ))
                     }
                 }