test(adkrest/models): add EventActions field mapping coverage

[nuthalapativarun]

Description of Change

Problem:
FromSessionEvent and ToSessionEvent were missing TransferToAgent, Escalate, and SkipSummarization fields in the REST API EventActions struct (issue #509). The fix was merged in #609, but there were no tests to prevent regressions.

Solution:
Add table-driven tests covering all three fields for both FromSessionEvent and ToSessionEvent, plus a round-trip test (FromSessionEvent → ToSessionEvent) to verify no EventActions fields are lost.

Closes #509

Testing Plan

Unit Tests:

• Added TestFromSessionEvent_EventActions — 5 table cases (all fields, each field individually, zero value)
• Added TestToSessionEvent_EventActions — same 5 cases for reverse direction
• Added TestFromSessionEvent_RoundTrip — full round-trip test
• All unit tests pass locally

$ go test ./server/adkrest/internal/models/...
ok  	google.golang.org/adk/server/adkrest/internal/models	0.209s

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.
• New and existing unit tests pass locally with my changes.

Additional context

This PR adds regression test coverage only — no production code changes. The underlying fix was already merged in #609.

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Authored by Varun Nuthalapati using AI coding assistance (Claude)

[nuthalapativarun]

Hi, just checking in on this one — happy to make any adjustments if something needs to change. Let me know if there's anything blocking review. Thanks!