fix(wallet): encode certificate ciphertext as Base64 JSON

[rohenaz]

Summary

• encode BRC-100 certificate field ciphertext as canonical padded Base64 instead of interpreting arbitrary bytes as UTF-8
• decode through CertificateCiphertext(base64:maximumByteCount:) so limits apply to decoded ciphertext bytes
• cover acquire-result, list-result, and prove-request JSON paths without changing acquire request or keyring revealer behavior

Regression evidence

• exact bytes 00 ff 80 01 7f a5 42 c3 encode as AP+AAX+lQsM= and decode byte-for-byte
• exact canonical acquire-result JSON is asserted
• a signed BRC-52 certificate retains the same unsigned preimage and verifies after JSON transport
• the 8-byte limit accepts a 12-character Base64 value while rejecting a 9-byte value with the same 12-character encoded length

Verification

• swift test --filter WalletBRC100JSONCodecTests — 12 passed
• swift test — 822 Swift Testing tests passed; XCTest suites also passed; external Go-oracle tests report their existing environment-based unavailable skips when BSV_GO_SDK_PATH is unset
• git diff --check