feat: add support for Firestore Pipeline

packages/google_cloud_firestore/README.md

@@ -160,6 +160,92 @@ for (final doc in querySnapshot.docs) {
 }
 ```
 
+### Firestore Pipeline Operations
+
+Firestore Pipeline operations are available for Firestore Enterprise edition
+databases. They are server-side queries for projections, expressions,
+aggregates, and vector search.
+
+```dart
+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:
+
+```dart
+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:
+
+```dart
+final expression = Expression.field('createdAt')
+    .timestampSubtract('day', 7)
+    .timestampToUnixSeconds()
+    .as('createdSeconds');
+```
+
+Expressions can be selected with an alias, used in filters, passed to aggregate
+stages, or composed with other expression helpers.
+
+#### E2E Testing
+
+Real-project Pipeline E2E tests live in `test/e2e/pipeline_e2e_test.dart`.
+They are skipped unless you provide a project and credentials:
+
+```bash
+export FIRESTORE_PIPELINE_E2E_PROJECT_ID="your-project-id"
+export FIRESTORE_PIPELINE_E2E_DATABASE_ID="your-enterprise-database-id"
+export GOOGLE_APPLICATION_CREDENTIALS="/path/to/service-account.json"
+dart test -P prod test/e2e/pipeline_e2e_test.dart
+```
+
+The E2E suite includes vector nearest-neighbor coverage. Create the vector index
+once for the test collection group before running the suite in CI:
+
+```bash
+gcloud firestore indexes composite create \
+  --project="your-project-id" \
+  --database="your-enterprise-database-id" \
+  --collection-group="pipeline_e2e_books" \
+  --query-scope=COLLECTION \
+  --field-config=field-path="runId",order=ASCENDING \
+  --field-config=field-path="embedding",vector-config='{"dimension":"3","flat":"{}"}'
+```
+
 #### Get All
 
 ```dart

packages/google_cloud_firestore/lib/google_cloud_firestore.dart

@@ -20,6 +20,8 @@ export 'src/firestore.dart'
         AggregateField,
         AggregateQuery,
         AggregateQuerySnapshot,
+        AliasedExpression,
+        BooleanExpression,
         BulkWriter,
         BulkWriterError,
         BulkWriterOptions,
@@ -39,12 +41,26 @@ export 'src/firestore.dart'
         ExplainMetrics,
         ExplainOptions,
         ExplainResults,
+        Expression,
         FieldMask,
         FieldPath,
         FieldValue,
         Filter,
         Firestore,
         GeoPoint,
+        Ordering,
+        Pipeline,
+        PipelineAggregateFunction,
+        PipelineAliasedExpression,
+        PipelineBooleanExpression,
+        PipelineExpression,
+        PipelineField,
+        PipelineFunctions,
+        PipelineOrdering,
+        PipelineResult,
+        PipelineSnapshot,
+        PipelineSource,
+        PipelineValueType,
         PlanSummary,
         Precondition,
         Query,
@@ -54,6 +70,7 @@ export 'src/firestore.dart'
         ReadOnlyTransactionOptions,
         ReadOptions,
         ReadWriteTransactionOptions,
+        Selectable,
         SetOptions,
         Settings,
         Timestamp,
@@ -67,8 +84,23 @@ export 'src/firestore.dart'
         WhereFilter,
         WriteBatch,
         WriteResult,
+        and,
+        ascending,
         average,
+        constant,
         count,
+        currentDocument,
+        descending,
+        equal,
+        field,
+        greaterThan,
+        greaterThanOrEqual,
+        lessThan,
+        lessThanOrEqual,
+        not,
+        notEqual,
+        or,
+        pipelineFunction,
         sum;
 export 'src/firestore_exception.dart'
     show FirestoreClientErrorCode, FirestoreException;

packages/google_cloud_firestore/lib/src/firestore.dart

@@ -45,6 +45,7 @@ part 'filter.dart';
 part 'geo_point.dart';
 part 'order.dart';
 part 'path.dart';
+part 'pipeline.dart';
 part 'query_partition.dart';
 part 'query_profile.dart';
 part 'rate_limiter.dart';
@@ -467,6 +468,13 @@ class Firestore {
     );
   }
 
+  /// Creates a [PipelineSource], which defines a Firestore Pipeline operation.
+  ///
+  /// Pipeline operations are available for Firestore Enterprise edition
+  /// databases and support server-side projections, expressions, aggregates,
+  /// and vector search.
+  PipelineSource pipeline() => PipelineSource._(this);
+
   /// Fetches the root collections that are associated with this Firestore
   /// database.
   ///