AIP-72: Add proper serialization for XCom and Variable responses

[kaxil]

Currently Xcom & Variables fail on main with following error:

ValidationError: 1 validation error for
tagged-union[StartupDetails,XComResult,ConnectionResult,VariableResult,ErrorResponse]
  Unable to extract tag using discriminator 'type' [type=union_tag_not_found, input_value={'key':
'test_key', 'value': 'test_value'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.10/v/union_tag_not_found

The entire stacktrace is:

decoder.get_message()
task_sdk/src/airflow/sdk/execution_time/task_runner.py:177: in get_message
    msg = self.decoder.validate_json(line)
/usr/local/lib/python3.9/site-packages/pydantic/type_adapter.py:446: in validate_json
    return self.validator.validate_json(
E   pydantic_core._pydantic_core.ValidationError: 1 validation error for tagged-union[StartupDetails,XComResult,ConnectionResult,VariableResult,ErrorResponse]
E     Unable to extract tag using discriminator 'type' [type=union_tag_not_found, input_value={'key': 'test_key', 'value': 'test_value'}, input_type=dict]
E       For further information visit https://errors.pydantic.dev/2.10/v/union_tag_not_found

This commit addresses serialization bug for XCom and Variable responses. Specifically:

• Added from_xcom_response and from_variable_response class methods in XComResult and VariableResult, respectively, to convert autogenerated API responses into runtime-compatible result models.

• Updated the supervisor’s handle_requests method to use these converters for GetXCom and GetVariable message handling.

• Adjusted tests in test_supervisor.py to verify the inclusion of the type field in serialized responses and ensure proper decoding using CommsDecoder.

• Improved test coverage by asserting that serialized responses match expected formats and verifying deserialization consistency.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[amoghrajesh]

Looks good, one comment