Fix max_steps=0 being silently ignored in run()

[meddadaek]

Closes #2458

max_steps = max_steps or self.max_steps treats max_steps=0 as
falsy, so passing max_steps=0 silently falls back to the agent's
default instead of actually running zero steps. Changed to an
explicit None check so 0 is respected.

[kuangmi-bit]

Bumping this for visibility — it's a 1-line real bug: max_steps=0 is silently ignored because max_steps or self.max_steps treats 0 as falsy. Closes #2458.

Tests pass. The fix is minimal — changed or to an explicit None check.

Also have 4 companion test PRs adding test coverage for existing issues (#2541, #2543, #2544, #2545) — all mergeable, zero conflicts. Happy to rebase any of them if needed.