fix(adkrest): accept functionCallEventId in run requests

[Zhang-986]

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Link to Issue or Description of Change

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

• Closes: POST /api/run_sse rejects functionCallEventId field sent by adk-web dev-ui #941

Problem:
The bundled ADK web UI sends functionCallEventId when resuming a long-running function call, but RunAgentRequest did not include that field. Because the REST decoder uses DisallowUnknownFields(), /api/run_sse rejected the request before the confirmation response could be processed.

Solution:
Add FunctionCallEventId to RunAgentRequest so the backend accepts the field while preserving strict unknown-field validation for other request keys. A regression test covers the decoder path used by /run_sse.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• go test ./server/adkrest/controllers
• git diff --check

Manual End-to-End (E2E) Tests:

Not run locally. The change is covered at the request decoder boundary that previously rejected the dev-ui payload.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• Relevant unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This keeps strict JSON decoding enabled and only adds the field already sent by the bundled web UI for long-running function responses.

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