fix(schema): raise a clear error for duplicate relationship types

[NathalieCharbel]

Description

Improves the SchemaValidationError raised for graph schemas that define the same relationship-type label more than once in relationship_types with different properties and constraints.
e.g.

"patterns": [
    {"source": "Patient",   "relationship": "PARTICIPATED_IN", "target": "Encounter"},
    {"source": "Encounter", "relationship": "PARTICIPATED_IN", "target": "Physician"}
  ],
  "relationship_types": [
    {"label": "PARTICIPATED_IN", "properties": [                                                                                                                                             
      {"name": "date"},
      {"name": "patient_name"},
      {"name": "encounter_name"}
    ]},
    {"label": "PARTICIPATED_IN", "properties": [                                                                                                                                             
      {"name": "encounter_name"},
      {"name": "physician_name"}
    ]}
  ],
  "constraints": [
    {"relationship_type": "PARTICIPATED_IN", "type": "KEY",
     "property_names": ["date", "patient_name", "encounter_name"]}
  ]

Currently, on schema validation, duplicate relationship-type labels are silently collapsed by last-write-wins lookup index. When a constraint then references a property that lived on the shadowed (overwritten) duplicate, validation fails with a misleading error that listed the surviving entry's properties as the only "valid" ones

  File "/workspace/.venv/lib/python3.12/site-packages/neo4j_graphrag/experimental/components/schema.py", line 708, in validate_constraints_against_node_types
    _validate_constraint_property_defined(
  File "/workspace/.venv/lib/python3.12/site-packages/neo4j_graphrag/experimental/components/schema.py", line 376, in _validate_constraint_property_defined
    raise SchemaValidationError(
neo4j_graphrag.exceptions.SchemaValidationError: KEY constraint references undefined property 'date' on relationship type 'PARTICIPATED_IN'. Valid properties: {'encounter_name', 'physician_name'}

This is incorrect on two counts: the property does exist (on the other duplicate), and - per Neo4j semantics - a relationship type is global per name, so two entries with the same label and different definitions are an invalid model in the first place.

GraphSchema now detects duplicate relationship_types labels up front and raises a clear, actionable SchemaValidationError that names the duplicated label, its property sets, the underlying Neo4j rule, and a suggestion to fix it. The check runs before constraint validation, so the clear duplicate-label error replaces the old misleading one.

This path intended for user-provided schema (where users should be informed about what's going wrong with their provided model). Follow-up PRs will handle the LLM schema-generation path, where the prompt will be slightly tuned to avoid having that + a post extraction validation step to reconcile automatically duplicate relationship types.

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Note

Please provide an estimated complexity of this PR of either Low, Medium or High

Complexity: Low

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

Checklist

The following requirements should have been met (depending on the changes in the branch):

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate