feat(firestore): support array_agg(), array_agg_distinct(), first(), last()

[Linchin]

TODO:

• verify docstring

[gemini-code-assist]

Summary of Changes

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This pull request significantly enhances the Firestore client library by introducing several new aggregation functions. These additions allow users to collect values into arrays, retrieve distinct values, and easily select the first or last elements within a dataset, thereby expanding the analytical capabilities of the query pipeline. The changes are thoroughly tested with both unit and system tests to ensure reliability.

Highlights

• New Aggregation Functions: Added support for array_agg(), array_agg_distinct(), first(), and last() aggregation functions within the Firestore pipeline expressions, enabling more powerful data manipulation.
• System Tests: Introduced comprehensive system-level end-to-end tests for the newly added aggregation functions to ensure their correct behavior in a real-world scenario.
• Unit Tests: Implemented dedicated unit tests for each new aggregation method to verify their individual functionality and integration with the existing expression framework.

Changelog

• packages/google-cloud-firestore/google/cloud/firestore_v1/pipeline_expressions.py
  • Added array_agg method to create an aggregation that collects values into an array.
  • Added array_agg_distinct method to create an aggregation that collects distinct values into an array.
  • Added first method to create an aggregation that selects the first value.
  • Added last method to create an aggregation that selects the last value.
• packages/google-cloud-firestore/tests/system/pipeline_e2e/aggregates.yaml
  • Added a new system test testArrayAgg to verify the array_agg function.
  • Added a new system test testArrayAggDistinct to verify the array_agg_distinct function.
  • Added a new system test testFirst to verify the first function.
  • Added a new system test testLast to verify the last function.
• packages/google-cloud-firestore/tests/unit/v1/test_pipeline_expressions.py
  • Added test_array_agg to validate the array_agg method's behavior.
  • Added test_array_agg_distinct to validate the array_agg_distinct method's behavior.
  • Added test_first to validate the first method's behavior.
  • Added test_last to validate the last method's behavior.

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[gemini-code-assist]

Code Review

This pull request introduces support for array_agg(), array_agg_distinct(), first(), and last() aggregation functions. The changes include updates to pipeline expressions and the addition of corresponding unit and system tests. My review found a couple of minor grammatical errors in the docstrings, for which I've provided suggestions. These comments do not conflict with any of the provided rules and are therefore kept as is.