Make Webhook URL secret

[chesster88]

This PR makes the url in the Webhook configuration a secure field. Addressing #430

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Note

Medium Risk
Touches notification delivery configuration by changing how webhook URLs are sourced and validated; mistakes could break existing webhook setups if secret resolution is misconfigured.

Overview
Makes the Webhook contact point url a secret/secure field. Webhook config parsing now always resolves url via decryptFn("url", ...), allowing the URL to be provided (or overridden) from secure settings and failing if the decrypted URL is empty.

Updates schema secret-field expectations and expands webhook config tests/fixtures to cover URL-from-secrets and URL override behavior.

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[chesster88]

Is it possible to review this PR please? It blocks our progress and we have 2 more repositories to propagate these changes to (terraform provider and crossplane provider).

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[chesster88]

just a friendly reminder about this PR