Use strict JWT part decoding and redacted errors

[fhwvtqdc2q-svg]

Summary

• reject JWT Base64URL parts with unexpected characters instead of ignoring them
• avoid echoing JWT strings or encoded JWT parts in error descriptions
• add tests for strict decoding and redacted error descriptions

Validation

• Not run locally: Swift toolchain is not installed on this Windows machine.
• Local checkout note: generated docs contain Windows-invalid paths, so the source was inspected from an archive excluding docs.

[NandanPrabhu]

Hi @fhwvtqdc2q-svg thanks for raising the PR. We will check this