feat: add zero() method to Summary (#481)

[Afnan-0206]

Summary

Adds .zero(labels) to Summary metrics, bringing API and feature parity with Histogram.prototype.zero().

Motivation

Issue #481 identified that Summary lacked the ability to initialize a known label combination to zero before observations occur.
This can be useful when applications want metric series to exist before the first observation.

Implementation

• Added Summary.prototype.zero(labels).
• Reused a shared createBaseValues() helper for Summary storage initialization.
• Added TypeScript declaration for Summary.zero().
• Added tests covering:
  • zero-valued quantiles
  • zero sum
  • zero count
  • export before observations
  • subsequent observations
  • resetting an existing label combination
  • invalid labels

Testing

• npx jest test/summaryTest.js
• npm run lint
• npm run check-prettier
• npm run compile-typescript
• npm test

All local validation passed.

Related Issue

Closes #481