fix: MessageConverter.toLlmResponse does not map ChatResponse Usage to LlmResponse

[hemasekhar-p]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: [google-adk-spring-ai] MessageConverter.toLlmResponse does not map ChatResponse Usage to LlmResponse #1159

Problem:
When using the adk-java-springai module, token consumption statistics are completely lost. The adapter's MessageConverter successfully parses text content but drops the metadata payload.

Solution:
Updated MessageConverter.toLlmResponse to intercept the Spring AI Usage metadata and map promptTokens, completionTokens, and totalTokens to the ADK's native GenerateContentResponseUsageMetadata.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added three new targeted test cases to MessageConverterTest which all pass successfully:

1. usageMetadataShouldBeEmptyWhenSpringAiMetadataIsNull
2. usageMetadataShouldBeEmptyWhenSpringAiUsageIsNull
3. usageMetadataShouldDefaultToZeroWhenSpringAiTokensAreNull

[ddobrin]

LGTM