fix: update register_printer to handle multiple compilation units

[sulphur-swarm]

Summary

Fixes #1780.

register_detector already iterates over self.compilation_units to create one detector instance per compilation unit. register_printer was missing this loop — it unconditionally created a single printer instance regardless of how many compilation units exist.

Changes

• slither/slither.py: Wrap printer instantiation in register_printer in a for compilation_unit in self.compilation_units: loop, mirroring the pattern used by register_detector.
• slither/printers/abstract_printer.py: Update AbstractPrinter.__init__ to accept a compilation_unit parameter as the first argument (mirroring AbstractDetector), and set self.contracts = compilation_unit.contracts instead of slither.contracts.
• slither/printers/inheritance/inheritance_graph.py: Update PrinterInheritanceGraph.__init__ to pass the compilation unit through to super().__init__().
• tests/e2e/printers/test_printers.py: Update tests to use register_printer instead of direct instantiation, which is the correct API.

Before

def register_printer(self, printer_class: type[AbstractPrinter]) -> None:
    _check_common_things("printer", printer_class, AbstractPrinter, self._printers)
    instance = printer_class(self, logger_printer)
    self._printers.append(instance)

After

def register_printer(self, printer_class: type[AbstractPrinter]) -> None:
    _check_common_things("printer", printer_class, AbstractPrinter, self._printers)
    for compilation_unit in self.compilation_units:
        instance = printer_class(compilation_unit, self, logger_printer)
        self._printers.append(instance)

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This fix was contributed with AI assistance by SwarmFix (sulphur-swarm).

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