feat: Binding url citation metadata for event

[flex-myeonghyeon]

Summary

• Bind URL citation metadata to events when processing model responses
• Extract and attach citation source URLs from grounding metadata to the corresponding event
• Enables downstream consumers to access citation information directly from events

Test plan

• Verify that citation metadata is correctly bound to events when URL citations are present
• Verify that events without citation metadata are unaffected
• Run existing unit tests to confirm no regressions

🤖 Generated with Claude Code

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https://github.com/flex-myeonghyeon/adk-java 로 dc81846cc58c367dc72dab0f0eb264a8e5b4cdaf 체리픽 하고 PR 올려줘

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