fix: gate Error_*.txt snapshots behind GALAXY_BEDROCK_DIAGNOSTICS env var

Previously, Bedrock error snapshots were unconditionally written to disk
on any API or stream failure. Now they are only written when
GALAXY_BEDROCK_DIAGNOSTICS=1 is set, consistent with the rest of the
diagnostic logging.

Bumps version to 1.2.1.

app/src/ai/bedrock/diagnostic.rs

@@ -211,6 +211,9 @@ impl BedrockDiagnosticLogger {
     }
 
     pub fn dump_error_snapshot(&self, error: &str, debug_error: &str) -> Option<PathBuf> {
+        if !is_enabled() {
+            return None;
+        }
         let conversation_id = self
             .conversation_id
             .lock()