Fix schedule_all chunking for small ranges

[fallintoplace]

Summary

Fix static_thread_pool::schedule_all batching so small non-empty ranges always make progress when the pool has more workers than items. The batching code now asserts the existing non-zero worker invariant, clamps non-empty chunk sizes to at least one item, and resets the pending task count after each intermediate flush.

Testing

• cmake --build build --target test.exec -j 8
• ./build/test/exec/test.exec "schedule_all on static_thread_pool handles ranges smaller than available parallelism"
• ./build/test/exec/test.exec "[types][static_thread_pool]"
• ./build/test/exec/test.exec

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[ericniebler]

/ok to test 4321ae4

[fallintoplace]

@ericniebler Clangformat is failing sadly, let me know if this needs an update from my side.

[ericniebler]

yes, you need to run clang-format over the two files you changed.

[ericniebler]

/ok to test ea73a5d