[Feature Request]: Upgrade to latest pycobertura

[gnpar]

Feature description

Pycobertura v4 added support for branch-coverage inspection, which is more accurate at reporting coverage than the previous line/statement only coverage report:

https://github.com/aconrad/pycobertura/blob/master/CHANGES.md#400-2025-04-13

For example, while this comment using coverage-action reports 100% coverage, reporting with the latest pycobertura version it's slightly below 100%:

Filename                                         Stmts    Miss  Cover    Missing
---------------------------------------------  -------  ------  -------  ----------------------
contracts/ETKLib.sol                                53       0  100.00%
contracts/EToken.sol                               119       0  100.00%
contracts/LPManualWhitelist.sol                     31       1  96.77%   ~64
contracts/Policy.sol                                38       1  97.37%   ~179
contracts/PolicyPool.sol                           188       1  99.47%   ~630
contracts/PolicyPoolComponent.sol                   11       0  100.00%
contracts/PremiumsAccount.sol                      128       4  96.88%   ~198, ~459, ~485, ~490
contracts/Reserve.sol                               62       0  100.00%
contracts/RiskModule.sol                            40       0  100.00%
contracts/interfaces/IEToken.sol                     0       0  100.00%
contracts/interfaces/ILPWhitelist.sol                0       0  100.00%
contracts/interfaces/IPolicyHolder.sol               0       0  100.00%
contracts/interfaces/IPolicyHolderV2.sol             0       0  100.00%
contracts/interfaces/IPolicyPool.sol                 0       0  100.00%
contracts/interfaces/IPolicyPoolComponent.sol        0       0  100.00%
contracts/interfaces/IPremiumsAccount.sol            0       0  100.00%
contracts/interfaces/IRiskModule.sol                 0       0  100.00%
contracts/interfaces/IUnderwriter.sol                0       0  100.00%
contracts/underwriters/FullTrustedUW.sol             2       0  100.00%
TOTAL                                              672       7  98.96%

Relevant discussion on this pycobertura issue: aconrad/pycobertura#167

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