feat: add metrics and usage to conversation streaming responses

[pjkroker]

Problem

Streaming conversation responses from the Bedrock Converse API do not expose model usage or latency information. Consumers have no way to track token consumption or response latency for conversation turns.

Issue number, if available: #2355

Related PRs (all required for #2355)

This feature spans 4 repositories. PRs should be merged in this order:

1. amplify-category-api — aws-amplify/amplify-category-api#3448 — Defines AmplifyAIMetrics/AmplifyAIUsage GraphQL types and updates resolvers
2. amplify-data — aws-amplify/amplify-data#697 — Adds TypeScript types and client-side deserializers
3. amplify-backend — aws-amplify/amplify-backend#3148 — Captures metrics/usage from Bedrock streaming response
4. amplify-codegen — aws-amplify/amplify-codegen#1008 — Adds fields to generated introspection metadata

Changes

• Added metrics (latencyMs) and usage (inputTokens, outputTokens, totalTokens) fields to the StreamingResponseChunk type, included in the final chunk when a turn completes.
• Extracted metrics and usage from the Bedrock metadata stream chunk in BedrockConverseAdapter.
• Serialized metrics and usage to JSON strings in ConversationTurnResponseSender before sending to AppSync.
• Added metrics and usage to the GraphQL message selection set in ConversationMessageHistoryRetriever.
• Updated the integration test schema with MockMetrics and MockUsage custom types on ConversationMessageAssistantResponse.
  Corresponding docs PR, if applicable:

Validation

• Added unit test: includes metrics and usage in the final streaming chunk (bedrock converse adapter)
• Added unit test: serializes metrics and usage to JSON when streaming (response sender)
• Updated existing streaming test assertions to account for the new metadata chunk
• All 77 unit tests in @aws-amplify/ai-constructs pass

Checklist

• If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
• If this PR requires a change to the Project Architecture README, I have included that update in this PR.
• If this PR requires a docs update, I have linked to that docs PR above.
• If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5c1bcbf

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@aws-amplify/ai-constructs	Minor

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