feat(builder): aggregate validation errors in StreamBuilder (#631)

[ZacLou]

Closes #631.

Summary

Instead of failing on the first invalid field, StreamBuilder now collects every problem and throws one ValidationError with a .issues[] array.

Changes

• Added ValidationError class (src/errors.ts) with a frozen issues[] array
• Modified all StreamBuilder setters to record issues instead of throwing immediately
• Missing-required-fields check now feeds into the same aggregated list
• _issues is reset after each failed build() so a fixed builder can be retried
• Added tests covering multi-field aggregation, issue reset, and happy path

Example

try {
  builder.token("bad").sender("").amount(-1).build();
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(err.issues);
    // ["token must be...", "sender must be...", "amount must be..."]
  }
}