DEV: improve error bedrock error messages (#454)

When bedrock rate limits it returns a 200 BUT also returns a JSON
document with the error.

Previously we had no special case here so we complained about nil

New code properly logs the problem

lib/completions/endpoints/aws_bedrock.rb

@@ -93,15 +93,23 @@ module DiscourseAi
         end
 
         def decode(chunk)
-          Aws::EventStream::Decoder
-            .new
-            .decode_chunk(chunk)
-            .first
-            .payload
-            .string
-            .then { JSON.parse(_1) }
-            .dig("bytes")
-            .then { Base64.decode64(_1) }
+          parsed =
+            Aws::EventStream::Decoder
+              .new
+              .decode_chunk(chunk)
+              .first
+              .payload
+              .string
+              .then { JSON.parse(_1) }
+
+          bytes = parsed.dig("bytes")
+
+          if !bytes
+            Rails.logger.error("#{self.class.name}: #{parsed.to_s[0..500]}")
+            nil
+          else
+            Base64.decode64(parsed.dig("bytes"))
+          end
         rescue JSON::ParserError,
                Aws::EventStream::Errors::MessageChecksumError,
                Aws::EventStream::Errors::PreludeChecksumError => e

lib/completions/endpoints/base.rb

@@ -135,6 +135,9 @@ module DiscourseAi
                   end
 
                   decoded_chunk = decode(chunk)
+                  if decoded_chunk.nil?
+                    raise CompletionFailed, "#{self.class.name}: Failed to decode LLM completion"
+                  end
                   response_raw << decoded_chunk
 
                   redo_chunk = leftover + decoded_chunk