Fix flaky MatrixPowerTest.test_basics by reading all generated shards

[shunping]

Flaky test (https://github.com/apache/beam/actions/runs/26220137898/job/77152699556):

_________________________ MatrixPowerTest.test_basics __________________________
[gw3] linux -- Python 3.11.15 /runner/_work/beam/beam/sdks/python/test-suites/tox/py311/build/srcs/sdks/python/target/.tox-py311-cloud/py311-cloud/bin/python

self = <apache_beam.examples.matrix_power_test.MatrixPowerTest testMethod=test_basics>

    def test_basics(self):
      matrix_path = self.create_temp_file(self.MATRIX_INPUT)
      vector_path = self.create_temp_file(self.VECTOR_INPUT)
      matrix_power.run((
          '--input_matrix=%s --input_vector=%s --exponent=%d --output=%s.result' %
          (matrix_path, vector_path, self.EXPONENT, vector_path)).split())
      # Parse result file and compare.
>     with open(vector_path + '.result-00000-of-00001') as result_file:
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E     FileNotFoundError: [Errno 2] No such file or directory: '/runner/_work/beam/beam/sdks/python/test-suites/tox/py311/build/srcs/sdks/python/target/.tox-py311-cloud/py311-cloud/tmp/tmp2mc8u76w.result-00000-of-00001'

apache_beam/examples/matrix_power_test.py:54: FileNotFoundError

The test was previously expecting the output to be written to a single file shard (.result-00000-of-00001). Depending on the runner execution details, the output can be split into multiple shards, resulting in a flaky FileNotFoundError.

This PR dynamically finds and reads all generated shards matching the prefix using glob, merging their contents before validating against the expected output.

Related to #38484

[gemini-code-assist]

Summary of Changes

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This pull request resolves a recurring flakiness issue in the MatrixPowerTest suite. By implementing a glob-based approach to locate and aggregate all output files matching the expected prefix, the test now correctly handles scenarios where the runner generates multiple shards, ensuring consistent and reliable validation.

Highlights

• Test Flakiness Resolution: Updated the MatrixPowerTest.test_basics method to dynamically read all generated output shards instead of relying on a single hardcoded filename, preventing FileNotFoundError in environments where multiple shards are produced.

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[gemini-code-assist]

Code Review

This pull request updates the matrix power integration test to handle multiple output shards by using the glob module to aggregate results instead of reading a single hardcoded file. The reviewer suggested adding an assertion to ensure that output shards are actually found, which improves debuggability by preventing the test from proceeding with an empty result set if the pipeline fails to generate output.

[shunping]

r: @derrickaw (another flaky test related to asserting text output with different number of shards)

[github-actions]

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