fix(introspection): coalesce concurrent cache misses

[shreemaan-abhishek]

Summary

Coalesces concurrent introspection cache misses for the same token so one request calls the endpoint and waiters share its result. Cache bypass remains independent, normal cache hits avoid lock overhead, and non-cacheable responses are shared only with the in-flight request generation.

Checklist

• Tests pass with docker build -f tests/Dockerfile . -t lua-resty-openidc/test
• Tests pass with docker run -t --rm lua-resty-openidc/test:latest
• ChangeLog is updated for behavior changes, bug fixes, features, or user-visible documentation changes

The focused introspection suite passes 71/71, and both complete CI runs pass.

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