Align VaporReference and ConcreteReference Nullability name() Formatting

## Issue Details

There is a behavioral inconsistency in how JSpecify `@Nullable` annotations are prepended to class names between `ConcreteReference` and `VaporReference` in the `gapic-generator-java` AST. This technical debt is captured in the code via // TODO(miraleung): Give this behavioral parity with ConcreteReference.

1. **`ConcreteReference.name()`** manually and dynamically prepends `@Nullable ` to the class name if `isNullable()` is true:
```java
if (isNullable()) {
sb.append("@Nullable");
sb.append(SPACE);
}
sb.append(simpleName());
```
2. **`VaporReference.name()`** does not include this check in its name formation. Instead, nullability prepending for `VaporReference` was implemented purely inside the writer phase in `JavaWriterVisitor.visit(VaporReference)`.

This inconsistency causes issues when reference names are dynamically constructed or compared (for example, when a `VaporReference` or `ConcreteReference` is nested as a generic parameter inside another reference, since the parent's `build()` method calls `r.name()` on its generics to build its own name string).

To unblock the initial JSpecify `@Nullable` PR, a temporary patch was applied to copy over the minimal behavior, but we need a clean, long-term alignment.

### Proposed Follow-up Work:
1. Add a dedicated golden test case capturing the behavior of nullable inner classes and generic types to ensure we prevent regressions.
2. Refactor `VaporReference` and `ConcreteReference` to use a consistent representation of nullability formatting in `name()`.
3. Investigate if we can simplify `ConcreteReference` by removing the manual string formatting inside the AST class entirely, centralizing the nullability formatting logic inside the `JavaWriterVisitor`.

---

## Environment

* **OS Type and Version:** Linux
* **Java Version and JDK Vendor:** Java 11+
* **Module:** `sdk-platform-java/gapic-generator-java`

---

## Dependencies

* `com.google.api:gapic-generator-java`

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## Reproducer

The issue can be reproduced by comparing the string returned by `name()` for a nullable `VaporReference` vs a nullable `ConcreteReference`:

```java
// ConcreteReference name will contain "@Nullable"
ConcreteReference.builder()
.setClazz(String.class)
.setIsNullable(true)
.build()
.name(); // Returns "@Nullable String"

// VaporReference name will NOT contain "@Nullable"
VaporReference.builder()
.setName("MyClass")
.setPakkage("com.google")
.setIsNullable(true)
.build()
.name(); // Returns "MyClass"
```

This causes formatting differences when these references are nested inside collection types (e.g. `List<@Nullable String>` vs `List`).

Start with ConcreteReference.name(), VaporReference.name(), and JavaWriterVisitor.visit(VaporReference) in the gapic-generator-java module to trace where nullable formatting is applied. Add golden coverage for nullable inner classes and generic types, then verify that nested reference names are formatted consistently.

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