Crash if duplicate model names (across files) used in allOf

A TSP conversion PR resulted in this error:

```
"details":"incompatible properties : tags

definitions/ContainerGroup/properties/tags
at file:///home/runner/work/azure-rest-api-specs/azure-rest-api-specs/specification/containerinstance/resource-manager/Microsoft.ContainerInstance/ContainerInstance/stable/2025-09-01/containerInstance.json#L2245:8

definitions/Resource/properties/tags
at file:///home/runner/work/azure-rest-api-specs/azure-rest-api-specs/specification/containerinstance/resource-manager/Microsoft.ContainerInstance/ContainerInstance/stable/2025-09-01/containerInstance.json#L4642:8"
```

- https://github.com/Azure/azure-rest-api-specs/actions/runs/23628050434
- https://github.com/Azure/azure-rest-api-specs/pull/41602

Root cause, oad cannot handle two definitions in different files with the same name, if one definition is used in an `allOf`:

### duplicate-names.json
```json
{
"swagger": "2.0",
"info": {
"title": "duplicate-names",
"version": "1.0"
},
"paths": {},
"definitions": {
"Bar": {
"type": "object",
"properties": {
"p1": {
"type": "string"
}
},
"allOf": [
{
"$ref": "./common-types.json#/definitions/Foo"
}
]
},
"Foo": {
"type": "object",
"properties": {
"p1": {
"type": "object"
}
}
}
}
}
```

### common-types.json
```json
{
"swagger": "2.0",
"info": {
"title": "common-types",
"version": "1.0"
},
"paths": {},
"definitions": {
"Foo": {
"type": "object",
"properties": {
"p1": {
"type": "string"
}
}
}
}
}
```

### test result
```
incompatible properties : p1
definitions/Bar/properties/p1
at file:///home/mharder/openapi-diff-mh/incompat-prop/src/test/specs/incompatible-properties/duplicate-names.json#L12:8
definitions/Foo/properties/p1
at file:///home/mharder/openapi-diff-mh/incompat-prop/src/test/specs/incompatible-properties/duplicate-names.json#L25:8
```

- Test case: https://github.com/Azure/openapi-diff/pull/481

Reason, it assumes model names are unique across all files:

https://github.com/Azure/openapi-diff/blob/bb4653d32f49ceb2abab48656c42c253865697e1/src/lib/util/resolveSwagger.ts#L346-L352

Related: #347