fix: encode large base64url buffers safely

[samsamtrum]

Summary

• avoid spreading every byte into String.fromCharCode when base64url-encoding buffers
• add coverage for encoding a large buffer that previously exceeded the JavaScript argument limit

Why

base64UrlEncode is used for WebAuthn challenges and accepted any ArrayBuffer, but String.fromCharCode(...bytes) throws a RangeError once the buffer is large enough because it passes every byte as a function argument. Building the binary string incrementally keeps the behavior the same for normal inputs while avoiding that argument-limit failure.

Test

• corepack pnpm vitest run packages/core/src/utils/utils.test.ts
• corepack pnpm biome check packages/core/src/utils/utils.ts packages/core/src/utils/utils.test.ts

[vercel]

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