Type checkers reject Event convenience kwargs (route=, message=, state=, node_path=) #7623

🔴 Required Information

Describe the Bug:

Event accepts message=, state=, route= and node_path= in its
constructor, and the workflow samples use them. They are not Pydantic fields:
the mode='before' validator _accept_convenience_kwargs moves them into
content, actions.state_delta, actions.route and node_info.path. Static
type checkers only see the fields, so pyright reports every such call as an
error. contributing/samples/workflows alone has 59 of them (35 message,
13 state, 11 route).

Because Event uses extra='ignore', a misspelled keyword such as
Event(rout='next') is also dropped silently at runtime. A type checker is the
only tool that could catch it, but today it cannot tell the typo from the valid
keywords.

Steps to Reproduce:

1. Install google-adk 2.12.0 (also reproduced on main 1def14b) and pyright
   1.1.407.
2. Save the snippet under "Minimal Reproduction Code" as typo_check.py.
3. Run pyright typo_check.py contributing/samples/workflows/route/agent.py.

Expected Behavior:

The four convenience kwargs type-check, and only the typo rout= is reported.

Observed Behavior:

contributing/samples/workflows/route/agent.py:28:16 - error: No parameter named "state" (reportCallIssue)
contributing/samples/workflows/route/agent.py:43:15 - error: No parameter named "route" (reportCallIssue)
contributing/samples/workflows/route/agent.py:60:7 - error: No parameter named "message" (reportCallIssue)
typo_check.py:3:7 - error: No parameter named "route" (reportCallIssue)
typo_check.py:3:21 - error: No parameter named "message" (reportCallIssue)
typo_check.py:3:35 - error: No parameter named "state" (reportCallIssue)
typo_check.py:3:51 - error: No parameter named "node_path" (reportCallIssue)
typo_check.py:4:7 - error: No parameter named "rout" (reportCallIssue)
8 errors, 0 warnings, 0 informations

mypy with the pydantic plugin accepts the convenience kwargs, but it also
accepts the typo, because Event allows extra keys.

Environment Details:

• ADK Library Version (pip show google-adk): 2.12.0
• Desktop OS: macOS
• Python Version (python -V): 3.11.15

Model Information:

• Are you using LiteLLM: N/A (no model call involved)
• Which model is being used: N/A

🟡 Optional Information

Minimal Reproduction Code:

from google.adk.events.event import Event

Event(route='next', message='hi', state={'k': 1}, node_path='a/b')
Event(rout='next')  # typo

How often has this issue occurred?:

• Always (100%)

Additional Context:

A fix is proposed in #7622: a TYPE_CHECKING-only __init__ that lists the
fields and the four convenience kwargs, with a unit test that keeps it in sync
with Event.model_fields.