[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:

https://github.com/OpenAPITools/openapi-generator/blob/2c7efda060f64aed0b3afd52fc2247e5180b35e0/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/JSON.mustache#L193-L201

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

[wing328]

thanks for the PR

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)

let's give it a try