feat: centralize operation inference, validate explicit vs inferred mismatch

[zemanlx]

Operation inference (CREATE/UPDATE/DELETE) now works consistently regardless of whether test fixtures are in separate files or consolidated in .request.yaml.

Changes:

• parseObjectYAML and parseOldObjectYAML no longer hardcode the operation (CREATE/DELETE). They build a partial AdmissionRequest with resource metadata only.
• New inferOrValidateOperation() runs after all files are merged:
  • Empty operation: inferred from object/oldObject presence
  • Explicit operation matching inference: accepted
  • Explicit operation conflicting with inference: error
  • Explicit operation with no objects (e.g. CONNECT): accepted
• InferOperation renamed to inferOperation (unexported, only used internally by inferOrValidateOperation).
• Added ErrOperationMismatch sentinel error.
• README Operations section updated to reflect that inference works the same way for both split files and .request.yaml.
• go fix applied (evaluator, reporter).