Fix bedrock ConverseStream undocumented /delta/stop_sequence

[gaspar09]

background

PR #11286 attempted to expose the stop sequence from Bedrock's Converse API, but had a bug: it was reading from the wrong response path. The AWS Bedrock API actually returns the stop sequence nested under a delta object: additionalModelResponseFields.delta.stop_sequence, not directly at additionalModelResponseFields.stop_sequence.

summary

Fixed stop sequence extraction to read from correct API response path

• Changed extraction path from additionalModelResponseFields.stop_sequence to additionalModelResponseFields.delta.stop_sequence
• Updated TypeScript schema to include delta wrapper in BedrockAdditionalModelResponseFieldsSchema
• Updated tests to match actual AWS Bedrock API response structure
• Verified fix works with both generateText and streamText

implementation details

The AWS Bedrock Converse API returns additional model response fields nested under a delta object when using additionalModelResponseFieldPaths: ['/delta/stop_sequence']. The previous implementation incorrectly assumed the field would be at the top level of additionalModelResponseFields.

Correct structure:

{
  messageStop: {
    stopReason: "stop_sequence",
    additionalModelResponseFields: {
      delta: {
        stop_sequence: "END"  // ← nested under delta
      }
    }
  }
}

verification

Tested with streamText using stopSequences: ['END']. Confirmed providerMetadata.bedrock.stopSequence now correctly returns "END" when the stop sequence is triggered (previously returned undefined).

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Fixes: #11286

[vercel-ai-sdk]

⚠️ Backport to release-v5.0 created but has conflicts: #11414