Do not dispatch thread-pool bulk for negative shapes

[fallintoplace]

Summary

bulk_chunked treats non-positive shapes as empty ranges. The generic implementation already follows that rule, but the specialized thread-pool paths only checked whether the shape was nonzero.

• Avoid dispatching negative shapes to even_share in static_thread_pool.
• Avoid converting negative shapes into a huge agent count in thread_pool_base.
• Add regression coverage for both implementations.

Tests

• cmake --build build --target test.exec test.stdexec --parallel 4
• ./build/test/test.stdexec
• ./build/test/exec/test.exec

[copy-pr-bot]

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