Add tests for hash generation

[rahearn]

Refactors the rake task to enable testing of the changes made in #472

All PRs:

• Has tests
• Documentation updated

[Copilot]

Pull request overview

This PR refactors the hash generation logic from the rake task into a testable TaskHelper module, enabling comprehensive testing of the whitespace handling fix introduced in PR #472.

Key Changes:

• Extracted hash generation methods from lib/tasks/tasks.rake into a new TaskHelper module
• Added comprehensive test coverage for inline script/style hash generation
• Tests verify that helper tag indentation doesn't affect hash generation (the core fix from #472)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/secure_headers/task_helper.rb	New module containing extracted hash generation logic with methods for finding and hashing inline scripts/styles, previously embedded in the rake task
lib/tasks/tasks.rake	Refactored to use the new TaskHelper module, replacing inline method definitions with module inclusion
spec/lib/secure_headers/task_helper_spec.rb	New comprehensive test suite covering hash generation for scripts and styles, including tests for the indentation-independent hashing behavior

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[rei-moo]

@rahearn Looks good! Thanks very much for your contribution. Sorry it took so long for us to get around to reviewing it. 😄