Align key/id path validation for variables and connections

[henry3260]

Why

Before this PR, similar key/id parameters were handled differently:

• xcoms.py used Path(min_length=1) (request validation layer),
• variables.py used Path() plus manual if not variable_key checks (runtime layer),
• connections.py used Path() without a minimum-length constraint.

This inconsistency caused uneven behavior and error handling for the same class of inputs.

Was generative AI tooling used to co-author this PR?

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