With the upcoming deprecation of anon and service role keys, this PR
updates the OAuth server examples to use the secret and publishable keys
instead.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Documentation**
* Updated OAuth server authentication guides with revised client
initialization examples
* Improved environment variable references in code samples for server
client setup
* Enhanced troubleshooting sections with corrected terminology for
debugging Row-Level Security policy issues
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