Fix double serialization in XCom PATCH API endpoint

[Arunodoy18]

The update_xcom_entry endpoint was calling XComModel.serialize_value twice, causing values to be excessively serialized. This made the updated XCom values unusable when retrieved by tasks.

Changes:

• Removed duplicate XComModel.serialize_value call in update_xcom_entry
• Fixed existing test to remove double serialization in assertion
• Added comprehensive test to verify patched values are correctly serialized and can be retrieved/deserialized properly

Fixes: https://github.com/apache/airflow/issues/59032

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[Arunodoy18]

I have solved the issue , please assure reviewing the issue which is solved, If any thing is needed which is to be changed , please tell. Thank you .

[pierrejeambrun]

Needs rebase + conflict solving.