Fix broken import in TaskHandlerWithCustomFormatter

[jfmlima]

TaskHandlerWithCustomFormatter fails to import on Airflow 3.2.x because it references the private function _render_template_to_string from airflow.utils.helpers, which was removed in #54715.

This causes an ImportError at startup for any deployment using a custom logging config that references this handler:

from airflow.utils.helpers import _render_template_to_string, parse_template_string
ImportError: cannot import name '_render_template_to_string' from 'airflow.utils.helpers'

Uses the public render_template_to_string from airflow.sdk.definitions.context (where #54715 moved it) instead of the removed private function.

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

• Yes — Claude Code (Anthropic)

Generated-by: Claude Code following the guidelines

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