fix: use urlsafeB64Decode everywhere

[bshaffer]

fixes #596

TODO

• tests

[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical issue where JWT keys might not be correctly interpreted due to inconsistencies in base64 decoding. By standardizing the key decoding process to use urlsafeB64Decode across relevant methods, the change enhances the reliability and security of JWT operations, ensuring that keys are always processed as intended.

Highlights

• Key Decoding Consistency: Replaced base64_decode with self::urlsafeB64Decode in both the sign and verify methods to ensure consistent and correct decoding of keys, particularly for URL-safe base64 encoded strings.

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[gemini-code-assist]

Code Review

This pull request correctly addresses the issue of decoding base64url-encoded EdDSA keys by switching to urlsafeB64Decode. My main feedback is about code maintainability and testing. I've added comments regarding duplicated code that should be refactored. Additionally, as noted in the PR description, tests are still pending. It is critical to add tests that verify this fix with base64url-encoded keys to prevent regressions.