Add valid and invalid MappingSpecification YAML test data

Introduces new YAML files under tests/data/valid and tests/data/invalid to provide examples of both valid and invalid MappingSpecification objects. These files can be used for testing schema validation and error handling.

Author: matentzn

tests/data/invalid/MappingSpecification-001.yaml

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+# Invalid MappingSpecification - author is a string instead of Agent object
+---
+author: "Jane Smith"
+content: https://example.org/mappings/invalid.sssom.tsv
+subject_source:
+  name: Source X
+  content: https://example.org/source-x
+  content_type: text/plain
+object_source:
+  name: Source Y
+  content: https://example.org/source-y
+  content_type: text/plain

tests/data/invalid/MappingSpecification-002.yaml

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+# Invalid MappingSpecification - subject_source is a string instead of Source object
+---
+author:
+  id: example:author456
+  name: Test Author
+content: https://example.org/mapping.txt
+subject_source: "Source A"
+object_source:
+  name: Source B
+  content: https://example.org/source-b
+  content_type: text/plain

tests/data/invalid/MappingSpecification-003.yaml

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+# Invalid MappingSpecification - creator is a list instead of Agent object
+---
+author:
+  id: example:author789
+  name: Another Author
+creator:
+  - John Doe
+  - Jane Smith
+content: https://example.org/mapping.txt
+subject_source:
+  name: Valid Source
+  content: https://example.org/source-valid
+  content_type: text/plain
+object_source:
+  name: Another Source
+  content: https://example.org/source-invalid
+  content_type: text/plain

tests/data/valid/MappingSpecification-001.yaml

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+# Valid MappingSpecification example - SSSOM mapping between two ontologies
+---
+publication_date: "2024-01-15"
+license: CC-BY-4.0
+version: "1.0.0"
+description: Mappings between disease ontologies DO and MONDO
+type: SSSOM
+mapping_method: manual curation
+documentation: https://example.org/do-mondo-mappings/docs
+content: https://example.org/mappings/do-mondo.sssom.tsv
+subject_source:
+  name: Disease Ontology
+  version: "2024-01-01"
+  type: owl ontology
+  documentation: https://disease-ontology.org/
+  content: http://purl.obolibrary.org/obo/doid.owl
+  content_type: application/rdf+xml
+object_source:
+  name: Mondo Disease Ontology
+  version: "2024-01-10"
+  type: owl ontology
+  documentation: https://mondo.monarchinitiative.org/
+  content: http://purl.obolibrary.org/obo/mondo.owl
+  content_type: application/rdf+xml

tests/data/valid/MappingSpecification-002.yaml

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+# Valid MappingSpecification example - R2RML mapping between database and RDF
+---
+
+publication_date: "2023-11-20"
+license: Apache-2.0
+version: "2.1.0"
+description: R2RML mapping from legacy patient database to FHIR RDF
+type: R2RML
+mapping_method: automated generation with manual review
+documentation: https://example.org/patient-fhir-mapping
+content: https://example.org/mappings/patient-db-to-fhir.r2rml.ttl
+subject_source:
+  name: Patient Database
+  version: "v5.2"
+  type: relational database
+  documentation: https://example.org/db-schema
+  content: postgresql://example.org:5432/patients
+  content_type: application/sql
+object_source:
+  name: FHIR RDF Ontology
+  version: "4.0.1"
+  type: rdf vocabulary
+  documentation: http://hl7.org/fhir/
+  content: http://hl7.org/fhir/fhir.ttl
+  content_type: text/turtle

tests/data/valid/MappingSpecification-003.yaml

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+# Valid MappingSpecification example - minimal required fields only
+---
+content: https://example.org/minimal-mapping.txt
+subject_source:
+  name: Source A
+  content: https://example.org/source-a
+  content_type: text/plain
+object_source:
+  name: Source B
+  content: https://example.org/source-b
+  content_type: text/plain