fix: Reject duplicate handler names at registration

[RaphaelFakhri]

Reject duplicate handler names at registration

Fixes #10

Registering two handlers with the same name on a Service, VirtualObject or Workflow silently replaced the first handler with the second. The decorator now raises a ValueError when a handler with the same name is already registered, which surfaces the mistake when the module is imported.

The error message follows the one Endpoint.bind raises for duplicate service names, for example Handler greet already exists in Greeter.

Testing

tests/duplicate_handlers.py covers Service, VirtualObject and Workflow, including a workflow main handler and a shared handler that use the same name.

PYTHONPATH=python python -m pytest tests/duplicate_handlers.py

The tests fail without the change (DID NOT RAISE ValueError, 3 failed) and pass with it (3 passed).

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[RaphaelFakhri]

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