[Python] Deflake TextIO footer test

[bvolpato]

Fixes #39639.

TextSinkTest.test_write_pipeline_footer let the runner choose its output shard count even though its assertions expect one trailing footer. Text sinks add the footer to every shard, so a multi-shard run leaves extra footer lines in the combined output.

Use one output shard for this test, matching the adjacent header test. Per-shard header and footer behavior remains covered by test_write_max_bytes_per_shard.

Reproduction

Setting num_shards=5 on current master reproduces the failure deterministically: four footer lines remain after the assertion removes the final footer.

Testing

• Python 3.13 Gradle/tox targeted test
• 20 consecutive targeted test runs
• Related footer, header, max-records-per-shard, and max-bytes-per-shard tests

---

• Linked the issue with Fixes #39639.
• No CHANGES.md entry needed for a test-only change.
• Change is small and does not require an ICLA.

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