feat(builder): aggregate StreamBuilder validation errors into ValidationError (closes #631)

[Ranjeet2063]

Summary

Aggregates all parameter and required-field problems encountered during stream configuration into a single ValidationError carrying a .issues[] array, instead of terminating prematurely on the first invalid setter call (closes #631).

Changes

• src/errors.ts:
  • Added ValidationIssue interface (field?: string, message: string).
  • Added ValidationError class extending Error with readonly issues: readonly ValidationIssue[] and human-friendly structured summaries.
  • Registered ValidationError in isConduitError().
• src/builder.ts:
  • StreamBuilder setters store raw inputs without throwing immediately on invocation, enabling clean fluent chaining.
  • Added public validate(): ValidationIssue[] method for non-throwing validation inspection.
  • build() evaluates all field rules in aggregate and throws ValidationError with all detected issues in .issues[].
  • toContractArgs() surfaces missing ratePerSecond via ValidationError.
• src/index.ts:
  • Re-exported ValidationError and ValidationIssue.
• src/tests/stream-builder-validation-aggregate.test.ts:
  • 10/10 new unit tests verifying multi-issue collection, missing/invalid field aggregation, chaining safety, and error summary formatting.

Verification

• 100% passing tests in src/tests/stream-builder-validation-aggregate.test.ts (10/10).
• Full regression suite verified across builder and error test files (48/48).
• Clean TypeScript typecheck (tsc --noEmit).
• Zero ESLint errors.
• Successful ESM and CJS production Rollup builds.

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