Move checks on 3WT legs in the 3WT add method to be consistent with core

[rolnico]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• A PR or issue has been opened in all impacted repositories (if any)

Does this PR already have an issue describing the problem?
No

What kind of change does this PR introduce?
Bug fix

What is the current behavior?
Checks on 3WT legs are done during the legAdder.add() method, which is not consistent with powsybl-core.
PR powsybl/powsybl-core#3429 in core will change the tests and restrain the exception management so current behaviour will fail the tests.

What is the new behavior (if this is a feature change)?
Checks are done during the 3WTAdder.add() method, which is consistent with powsyblo-core. Tests as in powsybl/powsybl-core#3429 will continue to pass.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
95.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud