Move template-field validation out of AnthropicAgentSessionOperator __init__

[bramhanandlingala]

Summary

AnthropicAgentSessionOperator.__init__ validated message and outcome
before Jinja templating runs, even though both are template_fields. A
templated outcome (e.g. outcome="{{ params.outcome }}") would crash
at DAG-parse time with AttributeError: 'str' object has no attribute 'keys', before rendering ever gets a chance to resolve it to a real dict.

Fix

providers/anthropic/src/airflow/providers/anthropic/operators/agent.py:

• __init__ now only does plain assignments.
• The two validation checks (message/outcome mutual exclusivity, and
  outcome requiring description + rubric) moved to the top of
  execute(), after templating has run.

Testing

providers/anthropic/tests/unit/anthropic/operators/test_agent.py:

• Updated test_requires_exactly_one_of_message_or_outcome and
  test_outcome_requires_description_and_rubric to construct the operator
  first, then assert the ValueError is raised on .execute() instead of
  at construction.
• Added test_init_does_not_validate_message_or_outcome to lock in that
  __init__ accepts any combination without raising.

Exemption list

Removed AnthropicAgentSessionOperator's entry from
scripts/ci/prek/validate_operators_init_exemptions.txt, as required by
the burn-down rule.

Related to #70296

Gen-AI disclosure: I used a generative AI tool to help identify the root
cause, write tests, and draft the PR description. I reviewed, tested, and
verified all changes locally before submitting.

Was generative AI tooling used to co-author this PR?

• Yes — Claude

Generated-by: Claude following the guidelines

[shahar1]

See #70437 (review)

Concerns addressed