fix: align power electronics constraints with API

[SoheylM]

Description

Align the Power Electronics v0 problem with the constraint API described in the current documentation.

• Add THEORY error constraints for positive passive-component values, a fractional duty cycle, and binary switch levels.
• Add an IMPLEMENTATION error constraint for the PWL timing range imposed by the 10 ns transition time and 5 us switching period.
• Preserve the existing v0 design space as the supported benchmark and dataset domain.
• Warn and return non-finite objectives when ngspice output is invalid, allowing batch generation to continue.
• Remove switch-model parameters ignored by ngspice.
• Preserve dataclass fields with init=False when checking constraints against an existing problem configuration.
• Correct the constraint-category example in the developer documentation and document the Power Electronics constraints.

Part of #243 (Power Electronics).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Screenshots

Not applicable.

Validation

• pre-commit run --all-files
• ruff check .
• ruff format --check .
• Focused tests: 35 passed
• Broader tests excluding tests/test_problem_implementations.py: 78 passed, 9 skipped
• mypy passes for the affected source files
• A valid ngspice simulation returns finite objectives
• An invalid topology emits InvalidNgSpiceOutputWarning and returns NaNs without stopping the process

Local environment notes:

• The repository-wide mypy command currently reports pre-existing NumPy typing failures across 53 files.
• tests/test_problem_implementations.py cannot be collected locally because pytest-subtests is not installed in the project test environment.

Checklist

• I have run the pre-commit checks with pre-commit run --all-files
• I have run ruff check . and verified formatting with ruff format --check .
• I have run mypy .
• I have commented my code where the behavior is not self-explanatory
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove the fix is effective
• New and existing unit tests pass locally with my changes

The unchecked warning item is intentional: invalid ngspice output now raises a dedicated warning while preserving NaNs for batch processing. The unchecked test and mypy items are explained in the validation notes above.

Reviewer Checklist

• The content of this PR brings value to the community. It is not too specific to a particular use case.
• The tests and checks pass.
• The documentation is updated.
• The code is understandable and appropriately commented.
• There is no merge conflict.
• The changes do not break existing benchmark results.
• For bug fixes, the fix is robust.

[g-braeunlich]

Only some very minor suggestions!

[SoheylM]

@g-braeunlich Thanks for the review. I addressed the comments and CI is green. Are you happy for us to merge?