fix: Improve error visibility when JWK kid lookup fails

[Minterl]

Problem

When GetJSONWebKey fails to find a matching key in the JWKS response, the current error message is:

missing json web key

This is misleading in two ways:

It implies no keys exist at all, when keys may exist but none matched the token's kid
It gives no indication of what kid was being looked for, making debugging extremely difficult

In practice this error surfaces when CLERK_SECRET_KEY is misconfigured — the JWKS endpoint returns keys for a different instance, none of which match the token's kid. The opaque error message makes this nearly impossible to diagnose withthout a few hours of source diving and manual tracing with delve.

Fix

Distinguish between two failure modes and include the kid being searched for in both:

"other keys found that did not match" => indicates wrong Clerk instance, key rotation, or misconfigured secret key
"no keys returned from endpoint" => No keys returned at all → indicates JWKS fetch returned an empty response

To make these errors surfaceable via the "default" way to use this package, the middleware, I've also taken the liberty of attaching them to the request context before handing control to failure handlers.

tl;dr

Before:

  "missing json web key"

After (keys exist but none matched):

  "no json web key found for kid \"ins_xxx\": other keys found that did not match"

After (no keys returned):

  "no json web key found for kid \"ins_xxx\": no keys returned from endpoint"