OpenApiWalker doesn't construct JSON pointer correctly

[davidrea-MS]

Describe the bug
When OpenApiWalker calls OpenApiVisitorBase.Enter(), it encodes / to ~1 but neglects to encode the ~ char first, inside the ReplaceSlashes() helper function. This results in the OpenApiVisitorBase.PathString property occasionally generating an invalid JSON pointer, such as during any of the built-in validation rules.

This can be seen by creating a document with a validation error under a property that contains a '~' in the name, and looking at the resulting OpenApiError.Pointer.

OpenApi File To Reproduce

{
    "openapi": "3.0.0",
    "info": {
      "title": "Test",
      "version": "1.0.0"
    },
    "paths": {},
    "components": {
      "schemas": {
        "my~schema": {
          "type": "object",
          "properties": {
            "goodProp": {
              "type": "string"
            },
            "nullProp": null
          }
        }
      }
    }
  }

Expected behavior
When validated with ValidationRuleSet.GetDefaultRuleSet(), validator should return an OpenApiError with the Pointer set to #/components/schemas/my~0schema/nullProp.
Instead, the Pointer is currently set to #/components/schemas/my~schema/nullProp, which is not a valid pointer.

Why is this a problem
The current format where tildas are not escaped is ambiguous. There's no way to distinguish between an encoded / and a literal ~1 string.
This prevents other tooling from being able to use the JSON pointer to highlight problems in the document.