Fix ObjectOutput handling of top-level scalar responses

[cfcromn]

Fixes #3860

Summary

ObjectOutput initialized its top-level result with Collections.emptyList().
When a top-level scalar response (for example, a bulk string returned from
EVAL with ScriptOutputType.OBJECT) was decoded, setValue(...) attempted
to append to the immutable list, resulting in an UnsupportedOperationException.

This change lazily initializes a mutable list when the first top-level scalar
value is received, preserving the existing behavior for nested collections.

Tests

• Added regression tests for top-level bulk string responses
• Added regression tests for top-level boolean responses
• Added regression tests for top-level double responses

Validation

• mvn test -Dtest=ObjectOutputUnitTests
• git diff --check

---

Note

Low Risk
Small, localized change to RESP3 decoding with new unit tests; nested map/list behavior is unchanged.

Overview
Fixes top-level scalar RESP3 responses (e.g. bulk strings from EVAL with ScriptOutputType.OBJECT) that previously crashed with UnsupportedOperationException because the decoder tried to append the first value to the immutable placeholder list from Collections.emptyList().

ObjectOutput now assigns the first decoded value directly to output and marks initialization complete, instead of creating an ArrayList on integer set() and appending. Nested arrays/maps still use the existing multi / multiMap paths unchanged.

Regression coverage adds unit tests for top-level bulk string, boolean, double, and integer decoding via RedisStateMachine.

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[a-TODO-rov]

Shouldn't we assign the actual value here ?

[cfcromn]

Thanks! Updated accordingly.

Top-level RESP3 scalar values are now assigned directly to output instead of being wrapped in a List.

[zhengjiudan]

If value is non-collection type, could we not initialize output as a List? Instead, just assign value to ouput?

[cfcromn]

Thanks for the suggestion!

Updated the implementation to assign top-level non-collection RESP3 values directly to output.

Nested collection responses are unchanged because multi() / multiMap() still initialize the collection container before nested values are accumulated.

I also added regression tests covering top-level bulk string, boolean, double, and integer responses.

[zhengjiudan]

A small comment