Fix Cloud Build operator template preparation

[abhisek343]

CloudBuildCreateBuildOperator stores a second copy of the templated build argument in build_raw during construction. This additional value read requires the operator to remain exempt from template-field constructor validation.

This change removes build_raw and uses a local build_path inside prepare_template() while loading JSON or YAML into self.build. Existing template-loading behavior remains unchanged, while the constructor now only assigns the argument to its corresponding instance field.

The obsolete constructor-validation exemption is removed in the same change.

related: #70296
cc @potiuk — taking CloudBuildCreateBuildOperator from the burn-down list.

Validation

• Cloud Build operator unit tests: 36 passed
• Operator constructor validation: passed
• Ruff formatting and lint checks: passed
• Git diff whitespace validation: passed

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

• Yes — OpenAI Codex

Generated-by: OpenAI Codex following the guidelines

[boring-cyborg]

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[olayiwola-gbenga]

Hi @abhisek343 please look into the failing checks

[shahar1]

Closing in favor of #70794 (included the first commit which is attributed to this PR's autthor)