fix: avoid exception-based Base64 decoder selection

[arnabnandy7]

• An issue was opened and discussed before writing the code
• Tests and linting pass
• Code coverage does not decrease; regression tests were added for Base64url decoding
• Appropriate documentation was updated

Fixes #2159 ☕️

This change avoids using exceptions to select between standard Base64 and Base64url decoders. It normalizes trailing whitespace, detects Base64url-specific characters (- and _), and selects the appropriate decoder directly.

Regression tests cover padded, unpadded, and trailing-whitespace Base64url input.