feat: detect and report self-signed certificate error in tls connections

[Liam-Tait]

Add a more helpful error message when a "SELF_SIGNED_CERT_IN_CHAIN" error code occurs, guiding users on how to resolve the issue.

Adding an error message for this specific case makes sense to me as the error is unrecoverable much like the "ENOTFOUND" case.

This error is unrecoverable since ioredis cannot connect unless the TLS
options are changed.

Without this change the error received by the user is the fastify plugin timeout message.
I personally came across this because Heroku Key-Value Store uses self-signed certificates.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Liam-Tait]

Open to a suggestion of how you would like this to be tested, if needed.
Documentation does not need to be changed IMO.

[Copilot]

Pull Request Overview

This PR adds a more helpful error message when encountering self-signed certificate issues in TLS connections, specifically targeting the SELF_SIGNED_CERT_IN_CHAIN error code. Instead of letting the plugin timeout, users will receive guidance on how to resolve the certificate issue.

• Adds specific error handling for SELF_SIGNED_CERT_IN_CHAIN error code
• Provides actionable guidance on TLS configuration options
• Treats this as an unrecoverable error similar to existing ENOTFOUND handling

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[jsumners]

A test is missing.

[jsumners]

Looks good to me.

[is2ei]

LGTM!

[Uzlopak]

LGTM