fix(events): avoid mutating event validation input

[VectorPeak]

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Link to Issue or Description of Change

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

• Closes: Event.model_validate mutates caller-provided input dict #6315
• Related: N/A

2. Or, if no issue exists, describe the change:

Problem:
Event._accept_convenience_kwargs() routes convenience inputs such as message, state, route, and node_path into the model's canonical nested fields. It currently does that by calling pop() directly on the data mapping received by the mode="before" model validator.

The before validator receives a dict in both common entry paths:

• Event(message="Hello!", state={"key": "value"}) receives a temporary dict assembled for model construction, so consuming convenience keys there stays internal.
• Event.model_validate(payload) receives the caller-provided dict itself. In that path, the validator mutates the original payload by replacing the convenience keys with content, actions, and node_info.

That second path is the bug: the issue is not just that a dict can theoretically reach the validator, but that the public model_validate(dict) API can hand the validator the same mapping object still owned by the caller.

A minimal reproduction before this fix:

payload = {
    "message": "Hello!",
    "state": {"key": "value"},
    "route": "next",
    "node_path": "root.node",
}
Event.model_validate(payload)
print(payload)

Before the fix, payload is rewritten in place. That can surprise callers that reuse the same mapping for logging, retries, comparisons, or subsequent validation.

Solution:
Copy the incoming mapping before consuming convenience keys. This keeps the existing convenience-key routing behavior unchanged while ensuring Event.model_validate() does not mutate caller-owned input.

The change is intentionally narrow:

• message still routes to content
• state still routes to actions.state_delta
• route still routes to actions.route
• node_path still routes to node_info.path
• only the caller-owned input dictionary is preserved

Testing Plan

Unit Tests:

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

Passed targeted unit tests locally:

D:\ZXY\Github\adk-python\.tox\py312\Scripts\python.exe -m pytest tests/unittests/events/test_event.py::TestMessageSerialization::test_model_validate_does_not_mutate_input_dict -q
1 passed, 4 warnings in 1.25s

D:\ZXY\Github\adk-python\.tox\py312\Scripts\python.exe -m pytest tests/unittests/events/test_event.py -q
48 passed, 7 warnings in 1.25s

Additional local checks:

uvx --from ruff==0.15.17 ruff check src/google/adk/events/event.py tests/unittests/events/test_event.py
All checks passed!

uvx --from pyink==25.12 pyink --check src/google/adk/events/event.py tests/unittests/events/test_event.py
2 files would be left unchanged.

git diff --check
passed

Manual End-to-End (E2E) Tests:

Not applicable for this validator-level change. The regression is covered by a focused unit test that exercises the public Event.model_validate() path and verifies both behaviors: the caller-provided payload remains unchanged, and the resulting Event still receives content, actions.state_delta, actions.route, and node_info.path.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This PR does not remove the existing pop()-based convenience-key routing. It only makes the validator operate on a local copy, so the canonical event output remains unchanged while avoiding a top-level mutation of caller-provided dictionaries.