Schema transformers receive an ignored property's metadata for a polymorphic discriminator node

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

`OpenApiSchemaService` emits the type discriminator of a polymorphic derived type as a one-value string constant, which is correct. When it hands that node to an `IOpenApiSchemaTransformer`, it populates `OpenApiSchemaTransformerContext.JsonPropertyInfo` (and therefore `JsonTypeInfo`) by matching the discriminator's name against the type's JSON metadata. That match can land on a property the emitter itself excluded from the schema because it carries `[JsonIgnore]`.

`context.JsonTypeInfo.Type` then reports the CLR type of an ignored property instead of the type of the node the framework actually built. A transformer that keys on the node's type rewrites the discriminator and silently corrupts it.

The same component already handles the exclusion correctly elsewhere. An ignored property on a non-polymorphic type produces no schema node and no transformer callback. Only the discriminator lookup reaches back into the excluded metadata.

### Expected Behavior

The transformer should not receive an ignored property's metadata for the discriminator node. Either no `JsonPropertyInfo` is supplied for a synthesized discriminator, or the lookup skips the properties the emitter excluded, so `context.JsonTypeInfo.Type` describes the node the framework built.

The control output is the correct document, and it is what a transformer has to be able to leave intact.

### Steps To Reproduce

[aspnetcore-openapi-discriminator-repro](https://github.com/iPilot/aspnetcore-openapi-discriminator-repro)

### Exceptions (if any)

_No response_

### .NET Version

10.0.400

### Anything else?

This block is byte-identical on `release/9.0`, `release/10.0` and `main`, so the .NET 11 preview line — `Microsoft.AspNetCore.OpenApi 11.0.0-preview.7`, which takes `Microsoft.OpenApi [3.9.0, 4.0.0)` — carries it unchanged. I read that from source rather than running it: 3.x is consumed only by the net11.0 packages and I have no .NET 11 SDK to hand.

### Suggested fix
Skip, in that loop, the candidates the emitter itself skipped. A JsonPropertyInfo with no accessors is exactly an ignored one:
```
foreach (var propertyInfo in jsonTypeInfo.Properties)
{
// the schema carries no node for an ignored property, so its metadata describes no node here
if (propertyInfo is { Get: null, Set: null })
{
continue;
}

...
}
```

Locate OpenApiSchemaService and the loop that matches discriminator names against jsonTypeInfo.Properties. Compare its candidates with the emitter's ignored-property handling, then use the linked aspnetcore-openapi-discriminator-repro to verify ignored metadata is not supplied to the transformer and the control document remains unchanged.

Written by the indexing model from the issue text.