Reject invalid Base64 padding in Bytes decoding

[haoshengzhen]

Summary

This PR makes Bytes.base64Decode() reject Base64 inputs that contain = padding before the end of the input.

Previously, base64DecodeLookup() accepted = as a valid character anywhere in the input, while the base64Decode() docs warned that padding in the middle could cause unexpected decoding results. The decoder now enforces that padding can only appear in the final two positions and that padding is contiguous once it starts.

Changes

• Add Base64 padding validation before decoding.
• Add a unit test for padding before the end of the input.
• Update the changelog under Unreleased / Fixed.

Verification

• npm run format:check -- CHANGELOG.md src/lib/provable/bytes.ts src/lib/provable/test/base64.unit-test.ts
• npx prettier --config .prettierrc.md.cjs --check CHANGELOG.md
• git diff --check

[haoshengzhen]

@martyall @Trivo25 Hi, Could you please review this PR at your convenience? Thank you very much.