Refactor: Move CspNonceProvider to Model and deprecate old Helper class

[mimou78]

Summary

This pull request refactors the CspNonceProvider by moving the existing implementation from the Helper namespace to the Model namespace, following Magento architectural best practices.
The original helper class is now deprecated and acts as a backward-compatible proxy, ensuring no breaking changes for third-party modules.

No functional behavior has been modified. CSP nonce generation and CSP header enrichment work exactly as before.

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Related Issue

Fixes #40266

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Problem

The class Magento\Csp\Helper\CspNonceProvider is incorrectly located in a Helper namespace while it functions as a service and not a view helper.
This misalignment introduces architectural inconsistencies and may lead developers to misinterpret its intended usage.

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Solution

• Added a new class: Magento\Csp\Model\CspNonceProvider, which now contains the complete implementation.
• Updated the old helper class to:
  • Include a @deprecated PHPDoc annotation (soft deprecation).
  • Proxy all logic to the new model class.
  • Trigger a deprecation notice for developers.
• Ensured full backward compatibility by keeping the helper class as a fallback.
• No runtime or functional changes introduced.

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Deprecation Notice

Magento\Csp\Helper\CspNonceProvider is now deprecated.
Developers should migrate to Magento\Csp\Model\CspNonceProvider.

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[ct-prd-pr-scan]

The security team has been informed about this pull request due to the presence of risky security keywords. For security vulnerability reports, please visit Adobe's vulnerability disclosure program on HackerOne or email [email protected].

[mimou78]

@magento run all tests