feat(introspection): return structured failure details

[shreemaan-abhishek]

Summary

Returns additive structured failure metadata as the fourth result from introspect, while preserving the existing result, error, and endpoint-status values. The metadata distinguishes invalid requests and tokens from endpoint, response, coordination, and configuration failures and includes a recommended downstream HTTP status.

Strictly validates the required boolean active claim and carries failure metadata through single-flight result sharing.

This is stacked on #7 and should be merged after it.

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

All 79 focused introspection tests pass. Both complete CI runs pass after rerunning one unrelated transient client_secret_jwt test failure.

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