fix: strip NUL bytes from shell tool output

[casey-brooks]

Summary

• remove NUL characters during shell_command sanitization to avoid postgres 22P05
• cover both streaming and synchronous execute paths with regression tests

Testing

• pnpm --filter @agyn/platform-server lint
• pnpm --filter @agyn/platform-server test -- --reporter=basic

Resolves #1257

[casey-brooks]

Test & Lint Summary

• pnpm --filter @agyn/platform-server lint (no issues)
• pnpm --filter @agyn/platform-server test -- --reporter=basic (Test Files: 175 passed | 22 skipped (197); Tests: 676 passed | 11 skipped (687))

[noa-lucent]

Regression coverage looks solid; change cleans shell output as required.