feat(type): scored unions

[marcj]

This adds support for simple scored unions. Previously the serializer picked the first matching union member. Now it takes a score per member and uses the one with highest score.

type T = {a: string} | {a: string, b?: number}

// Previously this was always picking first, leading to drop of b. Now it work.
const data = serialize<T>({a: 'foo', b: 123}); 

[Copilot]

Pull Request Overview

This PR adds support for scored unions to improve serialization behavior when union members have overlapping properties. Previously, the serializer always picked the first matching union member, which could lead to data loss. The new implementation assigns scores to union members and selects the one with the highest score, ensuring more accurate serialization.

• Implements scored union selection for better union member matching
• Updates type guard functions to return scores instead of simple boolean values
• Adds comprehensive test coverage for various union scenarios with optional and additional properties

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/type/src/serializer.ts	Core implementation of scored union logic with type guard scoring system
packages/type/src/typeguard.ts	Updates type guard to handle score-based returns for root validation
packages/bson/src/bson-deserializer-templates.ts	BSON-specific implementation of scored union deserialization
packages/bson/src/bson-serializer.ts	Updates BSON serializer to use new union handling
packages/type/tests/serializer.spec.ts	Test cases for union serialization with overlapping members
packages/type/tests/type-spec.spec.ts	Test cases for union JSON serialization scenarios
packages/bson/tests/bson-parser.spec.ts	BSON-specific union serialization test cases
packages/bson/tests/type-spec.spec.ts	Reordered test assertions for null/undefined handling
packages/mongo/src/query.resolver.ts	Added empty query handling for deterministic count results
packages/mongo/src/client/options.ts	Added estimateCount option to CommandOptions interface

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