refactor: extract shared helper for useHashes env-var boilerplate

[tylerbutler]

Summary

• Extracts a shared isHashingEnabled(envVar) helper function to eliminate repeated multi-line JSDoc + useHashes getter boilerplate across 4 LeafWithFileStatDoneFileTask subclasses (CopyfilesTask, TypeValidationTask, GoodFence, DepCruiseTask)
• Each class's useHashes getter is now a one-liner delegating to the helper
• The consolidated JSDoc on the helper documents the hashing behavior once

Fixes #26802

Test plan

• pnpm build passes in build-tools/packages/build-tools
• pnpm test passes (137 tests) in build-tools/packages/build-tools
• CI passes

[github-actions]

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[Copilot]

Pull request overview

Refactors fluid-build leaf tasks to remove repeated useHashes env-var boilerplate by introducing a single helper that encapsulates the “enable hashing via env var == '1'” behavior.

Changes:

• Added isHashingEnabled(envVar) helper (with consolidated JSDoc) for env-var controlled hashing.
• Updated CopyfilesTask, TypeValidationTask, GoodFence, and DepCruiseTask to delegate useHashes to the helper.

[github-actions]

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