Move BashOperator template field logic from __init__ to execute

[Kunal8954]

Related to #70296

Summary

Moves the template-field-dependent logic in BashOperator.__init__ (determining whether bash_command is an inline command or a script file) into execute(), since template fields are only rendered after __init__ runs. Previously this check ran on the un-rendered Jinja expression.

Changes

• Removed self._is_inline_cmd computation from __init__
• Added equivalent is_inline_cmd computation inside execute(), using the already-rendered self.bash_command
• Removed BashOperator entry from validate_operators_init_exemptions.txt

[boring-cyborg]

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[Kunal8954]

I've opened a PR for BashOperator: #70429
Moved the _is_inline_cmd check out of init and into execute(), since it was operating on the un-rendered bash_command template field. Also removed the BashOperator entry from validate_operators_init_exemptions.txt.
This is my first contribution to Airflow, happy to make any changes if needed!