GenericData.containsKey() returns true for unset (null) declared fields, violating Map contract

### Description

PR #2151 introduced an override for `GenericData.containsKey(Object name)` that queries `classInfo.hasFieldInfo(fieldName)`. However, `ClassInfo.hasFieldInfo` checks only whether the `@Key` field is declared in class reflection metadata, rather than checking whether the field holds a non-null value on the instance.

### Impact & Broken Invariants

In `GenericData`, declared fields with `null` values are treated as absent from the map:
1. **`keySet()` / `entrySet()` Contradiction**:
On `new MyData()`, `containsKey("field")` returns `true`, but `get("field")` is `null`, `entrySet()` has size `0`, and `keySet().toString()` outputs `[]`.
2. **`Set.contains` vs `Iterator` Inconsistency**:
Because `java.util.AbstractMap.keySet().contains(k)` delegates to `Map.containsKey(k)`, `model.keySet().contains("field")` evaluates to `true`, while iterating over `model.keySet()` yields `0` elements.
3. **Client Breakages**:
Code patterns checking for field presence (such as pagination checks like `if (response.containsKey("pageToken"))`) now evaluate to `true` even when the server never populated the field.

### Reproduction

```java
public class ExampleModel extends GenericData {
@Key private String optionalField;
}

ExampleModel model = new ExampleModel();

// Prior to 2.2.0:
// model.containsKey("optionalField") == false

// In 2.2.0:
model.containsKey("optionalField"); // returns true!
model.get("optionalField"); // returns null
model.keySet(); // prints []
model.keySet().contains("optionalField"); // returns true while iterator is empty
```

### Proposed Fix

In `com.google.api.client.util.GenericData.java`, check whether the declared field value is non-null, matching `DataMap.containsKey()`:

```java
@Override
public final boolean containsKey(Object name) {
if (!(name instanceof String)) {
return false;
}
String fieldName = (String) name;
FieldInfo fieldInfo = classInfo.getFieldInfo(fieldName);
if (fieldInfo != null) {
return fieldInfo.getValue(this) != null;
}
if (classInfo.getIgnoreCase()) {
fieldName = fieldName.toLowerCase(Locale.US);
}
return unknownFields.containsKey(fieldName);
}
```

Start in com.google.api.client.util.GenericData.java and inspect containsKey alongside DataMap.containsKey and the referenced FieldInfo access. Verify the ExampleModel reproduction, then confirm declared null fields are absent while non-null declared and unknown fields retain the expected Map behavior.

Rédigé par le modèle d'indexation à partir du texte de l'issue.