Fix 32-bit zstdmt max job size allocation failure

[danyalahmed1995]

Fixes #4628.

Summary

Reduces the 32-bit zstdmt maximum job size from 512 MB / log 29 to 256 MB / log 28.

On 32-bit builds, the previous bound could allow a threaded compression configuration at max compression level to attempt an oversized allocation, matching the failure described in #4628.

This follows the issue reporter's diagnosis and proposed cap reduction.

Changes

• Lower ZSTDMT_JOBLOG_MAX on 32-bit builds from 29 to 28.
• Lower ZSTDMT_JOBSIZE_MAX on 32-bit builds from 512 MB to 256 MB.
• Add a bounds check in the fuzzer unit tests so ZSTD_c_jobSize reports the expected architecture-specific upper bound.

Validation

• git diff --check
• Verified with a temporary GitHub Actions workflow on linux/386 using Debian testing:
  • make clean
  • make -j$(nproc)
  • make check

The temporary validation workflow was kept out of this PR branch.