fix(auth): structure metadata discovery errors

[ASTion24]

Summary

• replace protected resource metadata discovery failures encoded as MetadataError(String) with structured AuthError variants
• preserve HTTP discovery errors as source
• expose invalid metadata reasons and exhausted authorization server candidates as fields
• box the detailed metadata reason to keep AuthError below Clippy's result_large_err threshold

Testing

• cargo test --all-features
• cargo clippy --all-targets --all-features -- -D warnings

Fixes #1265