fix: auto-compound fail when free balance is too low

[librelois]

What does it do?

This PR fix a bug that impact the auto-compound of staking rewards and add a dev test to reproduce the bug.

The bug is that the auto-compound logic check if stakable amount is high enough, but this check doesn't take into account the reserve, so in some corner cases the auto-compound fail silently, then the user receive less staking rewards than expected.

Old buggy logic: stakable amount is free- frozen
New logic (fix): stakable amount is free + reserve - frozen

What important points reviewers should know?

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

[github-actions]

Coverage Report

@@                     Coverage Diff                     @@
##           master   elois-fix-auto-compound      +/-   ##
===========================================================
+ Coverage   81.30%                    81.33%   +0.03%     
  Files         301                       301              
- Lines       85701                     85669      -32     
===========================================================
- Hits        69675                     69672       -3     
- Misses      16026                     15997      -29     

Files Changed	Coverage
/client/rpc/manual-xcm/src/lib.rs	88.00% (+25.33%)	🔼
/client/rpc/txpool/src/lib.rs	65.38% (+8.65%)	🔼
/client/rpc-core/txpool/src/types/content.rs	70.37% (+1.85%)	🔼
/pallets/parachain-staking/src/lib.rs	91.74% (+0.01%)	🔼
/pallets/parachain-staking/src/mock.rs	97.63% (-0.02%)	🔽
/pallets/parachain-staking/src/tests.rs	91.36% (-0.03%)	🔽

Coverage generated Thu Jul 4 10:57:53 UTC 2024