[Python] Validate FileBasedSink num_shards type

[goutamadwant]

Fixes #25860

FileBasedSink currently accepts a side input for num_shards, then fails later while processing the pipeline.

This validates num_shards in the constructor and raises a clear TypeError when it is not an int. The regression test covers AsSingleton.

Before: the constructor accepts AsSingleton as num_shards.

After: the constructor raises TypeError: num_shards must be an int.

Tests:

• python -m unittest -q apache_beam.io.filebasedsink_test — 15 passed
• python -m unittest -q apache_beam.io.textio_test — 117 passed

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[goutamadwant]

retest this please