fix: remove Buffer dependency from core runtime

Summary

• replace Node-specific Buffer usage with @better-auth/utils/base64 for standard Base64 and unpadded Base64URL
• measure execution and runner byte limits with TextEncoder instead of Buffer.byteLength
• add regression tests that run with Buffer unavailable
• add Unicode encoding coverage

Note: Base64.decode now throws on characters outside the Base64 alphabet instead of silently skipping them like Buffer did.

Testing

• pnpm --filter callscript test — 275 tests passed
• pnpm --filter callscript typecheck
• pnpm --filter callscript build
• Biome checks passed for all changed files

Closes #14