[java][okhttp-gson] fix: JSON deserialization fallback for String return types

[klin1344]

This change is a follow-up to #21115, the fix for kubernetes-client/java#4020.

The original change switched deserialization to be InputStream-based, which no longer
falls back to returning the raw String on JsonParseException when the return type
is String.class:

openapi-generator/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/JSON.mustache

Lines 193 to 201 in 2c7efda

	} catch (JsonParseException e) {
	// Fallback processing when failed to parse JSON form response body:
	// return the response body string directly for the String return type;
	if (returnType.equals(String.class)) {
	return (T) body;
	} else {
	throw (e);
	}
	}

We cannot implement the same fallback for InputStream-based deserialization because an
InputStream can only be consumed once. If JSON parsing fails, we cannot
rewind the stream to return the raw content.

Fix:
In ApiClient, check the return type before choosing the deserialization path. If it is String.class, use String-based JSON.deserialize(String, Type) with fallback behavior. For all other types, use the InputStream-based JSON.deserialize(InputStream, Type) for streaming which supports responses > 2GB.

This preserves backward compatibility for the fallback behavior - when JSON parsing fails and the return type is String.class, the raw response body is returned as a String instead of throwing JsonParseException.

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[klin1344]

cc @lhotari