Fix BigInt serialization in SDK responses

[comzzy-comzzy]

Summary

• Convert nested bigint values to decimal strings before grant read results leave the SDK.
• Export the formatter for callers that need to sanitize their own payloads.
• Update response types and cover large, nested values plus direct JSON.stringify() calls.

Testing

• node node_modules/typescript/bin/tsc --target ES2022 --module NodeNext --moduleResolution NodeNext --strict --skipLibCheck --noEmit src/utils/formatters.ts tests/utils/formatters.test.ts
• Manual runtime check confirmed a value above Number.MAX_SAFE_INTEGER keeps its exact decimal representation and serializes successfully.

The full test suite and project typecheck are currently blocked on main by a malformed package.json, a missing Rollup optional binary, and existing syntax errors in unrelated files.

Closes #155