fix: reject a zero thread count

[sankalpsthakur]

This PR rejects -j0 / --threads=0 during shell option processing instead of allowing an invalid task-manager size to reach the runtime.

The runtime requires a positive worker count in multithreaded builds. Add an explicit zero check alongside the existing numeric and upper-bound validation, with a focused tests/misc regression asserting a clean diagnostic and nonzero exit.

Closes #4927

Validation

• Added tests/misc/threads_zero.sh
• The branch diff contains only the shell validation and focused CLI regression
• Full Lean CI is the authoritative validation

AI assistance

AI tools assisted with issue triage, duplicate-PR search, implementation, and regression preparation. I reviewed the shell option path and final two-file diff.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 05a6ddfb52c033f12967c96443a93b0ca7b8a58a --onto 5b8d06c1764fa367a1e27b3e89f64667eac425af. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-02 06:40:37)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 945e78b86645b179655123cd2fdba83d89d28d07 --onto 110db9cb751afaee8b2ac344887d6c7e632f77b4. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-04 02:10:40)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 05a6ddfb52c033f12967c96443a93b0ca7b8a58a --onto 23393b959b33e3a8d15796b2397f8a04c315b9f4. You can force reference manual CI using the force-manual-ci label. (2026-08-02 06:40:39)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 945e78b86645b179655123cd2fdba83d89d28d07 --onto 23393b959b33e3a8d15796b2397f8a04c315b9f4. You can force reference manual CI using the force-manual-ci label. (2026-08-04 02:10:42)