feat: add support for Firestore Pipeline

[Lyokone]

Add support for Firestore Pipeline

How to use

final snapshot = await firestore
    .pipeline()
    .collection('books')
    .where(Expression.field('active').equalValue(true))
    .sort([Expression.field('price').ascending()])
    .select([
      Expression.field('title'),
      Expression.field('price'),
      Expression.field('title').toUpperCase().as('upperTitle'),
      Expression.field('tags').arrayLength().as('tagCount'),
    ])
    .limit(10)
    .execute();
for (final result in snapshot.results) {
  print(result.data());
}

Aggregates

Aggregate stages use aliased aggregate expressions:

final snapshot = await firestore
    .pipeline()
    .collection('books')
    .where(Expression.field('active').equalValue(true))
    .aggregate([
      Expression.field('price').sum().as('totalPrice'),
      Expression.field('rating').average().as('averageRating'),
      PipelineFunctions.count().as('bookCount'),
    ])
    .execute();
final data = snapshot.results.single.data();
print(data);

Expressions

Use Expression.field, Expression.constant, and Expression.variable to
build expressions. Most helpers are also available as fluent methods:

final expression = Expression.field('createdAt')
    .timestampSubtract('day', 7)
    .timestampToUnixSeconds()
    .as('createdSeconds');

[gemini-code-assist]

Code Review

This pull request introduces Firestore Pipeline operations to the google_cloud_firestore package, enabling server-side projections, expressions, aggregates, and vector search, complete with comprehensive E2E and unit tests. The review feedback highlights several key improvement opportunities: reverting a breaking change to the DistanceMeasure enum by converting values to lowercase locally within the pipeline execution, optimizing performance by extracting a frequently compiled regular expression into a file-level constant, and adding as well as exporting missing top-level comparison helpers (lessThan and greaterThan) to ensure API completeness.

[github-actions]

Coverage Report

✅ Coverage 73.15% meets 40% threshold

Total Coverage: 73.15%
Lines Covered: 5859/8010

Package Breakdown

Package	Coverage
google_cloud_firestore	73.21%
firebase_admin_sdk	73.07%

Minimum threshold: 40%

[demolaf]

PipelineFunctions accepts Object? but only Expression.field() actually works without a fieldOrExpression-style check — untested since every call site already used field(). We should either commit to requiring Expression.field() explicitly or handle the coercion like Node's fieldOrExpression.