fix: handle PoXAnchorBlockRequired in boot_to_epoch_25_reward_cycle

[jacinta-stacks]

Fixes failing tests::signer::v0::multiple_miners_mock_sign_epoch_25. Similar problem was identified and fixed in boot_to_epoch_3. The test was panicking because get_reward_set_signers() returns a 400 PoXAnchorBlockRequired error when the prepare phase anchor block hasn't been determined yet. The original code used .expect() on this result, causing a hard failure on a transient condition. (Gives coordinator time to catch up)

[federico-stacks]

lgtm!

[francesco-stacks]

lgtm!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.09%. Comparing base (42fb45c) to head (d5b4923).
⚠️ Report is 143 commits behind head on develop.

❌ Your project status has failed because the head coverage (60.09%) is below the adjusted base coverage (84.96%). You can increase the head coverage or adjust the Removed Code Behavior.

❗ There is a different number of reports uploaded between BASE (42fb45c) and HEAD (d5b4923). Click for more details.

HEAD has 133 uploads less than BASE

Flag	BASE (42fb45c)	HEAD (d5b4923)
	292	159

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #6992       +/-   ##
============================================
- Coverage    84.96%   60.09%   -24.87%     
============================================
  Files          412      412               
  Lines       219958   219958               
  Branches       338      338               
============================================
- Hits        186883   132182    -54701     
- Misses       33075    87776    +54701     

see 313 files with indirect coverage changes

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[github-actions]

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