[Tech Debt] Warn when an action declares a return value

[vishnuprakaz]

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• I searched in the issues and found nothing similar.

Description

Actions communicate by sending events via ctx.send_event(...); any value an
action returns is silently discarded by the framework.

Today this fails quietly
if a user writes return result in an action, nothing happens and there is no
error or warning, which is a confusing trap.

python/flink_agents/plan/actions/action.py already carries a TODO for this:

Proposed fix:

when an agent plan is built, emit a warning for any @action-
decorated method that declares a non-None return type, pointing the user to
ctx.send_event(...). Implements the existing TODO; warning only, no change to
execution behavior

Are you willing to submit a PR?

• I'm willing to submit a PR!