BUG: update_node_formatters rejects documented string keys in pymc/model_graph.py

[aman-coder03]

Describe the issue:

The function update_node_formatters in pymc/model_graph.py validates the keys of node_formatters against the NodeType enum members. However, the public examples in model_to_graphviz and model_to_networkx show users passing string keys such as "Free Random Variable" and "Observed Random Variable".

Since these keys are strings (matching NodeType.value) and not actual NodeType enum members, the current validation raises a ValueError, even though the usage follows the documented examples.

This results in a mismatch between the documented API and the actual behavior.

Reproduceable code example:

### Reproducible Example


import pymc as pm

with pm.Model() as model:
    x = pm.Normal("x", 0, 1)

# Using documented string keys for node_formatters
node_formatters = {
    "Free Random Variable": lambda var: {"shape": "circle", "label": var.name},
}

# This raises a ValueError in update_node_formatters
pm.model_to_graphviz(model, node_formatters=node_formatters)

Error message:

PyMC version information:

• PyMC Version: 5.27
• Python Version: 3.12
• Operating System: Windows 11

Context for the issue:

No response

[aman-coder03]

@ricardoV94 shall i work on this?