fix: respect explicit zero custom_after_sleep in metamorph and reboot

[vdusek]

Description

• Actor.metamorph() and Actor.reboot() used a falsy check on custom_after_sleep, so an explicit timedelta(0) was treated as unset and silently replaced with the 5-minute default (Configuration.metamorph_after_sleep).
• Changed both checks to is None, so an explicit zero now skips the after-sleep entirely, while omitting the argument (or passing None) still uses the default.
• Added unit tests covering both methods, asserting that no sleep happens for timedelta(0) and that the underlying API call is still made.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.01%. Comparing base (2cc5602) to head (f6c047a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #971      +/-   ##
==========================================
+ Coverage   89.91%   90.01%   +0.09%     
==========================================
  Files          49       49              
  Lines        3085     3085              
==========================================
+ Hits         2774     2777       +3     
+ Misses        311      308       -3     

Flag	Coverage Δ
e2e	35.98% <0.00%> (ø)
integration	56.95% <0.00%> (-0.04%)	⬇️
unit	78.83% <100.00%> (+0.12%)	⬆️

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