fix: misc bugs found by AI

[akaladarshi]

While taking the code from the builtin actor into the fil-actor-state for the V18 actor update, coderabbit AI suggested some fixes/comments here.

I have fixed couple of them rest were about adding the checks for negative and zero but I verified most of them didn't find anything worth changing but will post in the comments if anybody thinks they are worth it. Also It turns out the changes were not that big.

[akaladarshi]

Here the document suggests

/// It is possible for balance to fall below the sum of PCD, LF and
/// InitialPledgeRequirements, and this is a bad state (IP Debt)

But the code actually doesn't allow the balance to fall below the sum of PCD, LF and InitialPledgeRequirements, so should the comments be updated.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.52%. Comparing base (68baa3c) to head (86a57a6).

Files with missing lines	Patch %	Lines
actors/power/src/state.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1733   +/-   ##
=======================================
  Coverage   90.52%   90.52%           
=======================================
  Files         139      139           
  Lines       27693    27695    +2     
=======================================
+ Hits        25069    25071    +2     
  Misses       2624     2624           

Files with missing lines	Coverage Δ
actors/evm/shared/src/uints.rs	98.35% <100.00%> (ø)
actors/evm/src/state.rs	85.71% <ø> (ø)
runtime/src/runtime/mod.rs	89.28% <ø> (ø)
runtime/src/runtime/policy.rs	100.00% <ø> (ø)
runtime/src/util/message_accumulator.rs	96.84% <ø> (ø)
runtime/src/util/set_multimap.rs	97.36% <100.00%> (+0.07%)	⬆️
actors/power/src/state.rs	86.84% <0.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[akaladarshi]

While validating power state validate_power_state here we are missing upper bound check on the self.faulty_power.qa.

Let me know if this is a real concern i can create a separate issue or change it in this PR.

[akaladarshi]

in amout_vested

let elapsed = curr_epoch - self.start_epoch;

could give us the negative value but I couldn't verify since the function is not used anywhere dead_code (should we remove this?)

[akaladarshi]

AddPairKey encoding is not implemented, we are not using it anywhere but still it panics.