Fix string array decode

[Prabal864]

Context / Problem

In Spring Framework 6 / WebFlux, using WebClient to decode a JSON array of strings
(e.g., ["hello", "world"]) into a List<String> sometimes resulted in unexpected behavior:
the JSON array was decoded as a single String containing the raw JSON ("["hello","world"]")
instead of a proper List<String>.

This issue also occurs for empty arrays: decoding "[]" into a List<String> could produce
a list containing a single string "[]" rather than an empty list.

This behavior is confusing and can lead to subtle bugs when consuming JSON arrays
from reactive endpoints.

---

Solution

This PR adds self-contained reproducer tests to the spring-webflux module
demonstrating the correct way to decode JSON arrays into List<String> using
ParameterizedTypeReference.

Key points:

1. Embedded Netty server for tests

   • The tests spin up a lightweight embedded Netty server on a dynamic port.
   • This ensures the tests are self-contained and do not rely on external services.

2. Correct JSON decoding

   • bodyToMono(new ParameterizedTypeReference<List<String>>() {}) correctly converts
     the JSON array into a List<String>.

3. Covers both empty and non-empty arrays

   • decodeNotEmptyList() → decodes ["hello", "world"] into a list of two strings.
   • decodeEmptyList() → decodes [] into an empty list.

4. Assertions

   • Uses AssertJ assertThat() to verify the decoded lists exactly match expected values.

---

Benefits

• Ensures WebClient correctly decodes JSON arrays into collections.
• Provides a reproducible test case for this edge case in WebFlux.
• Improves reliability for developers consuming JSON arrays in reactive applications.
• Fully self-contained; can run in CI without external dependencies.

---

Test Instructions

1. Run the tests locally:

   ./gradlew :spring-webflux:test --tests "org.springframework.web.reactive.function.client.StringArrayDecodeTests"

[rstoyanchev]

This is expected behavior documented at https://docs.spring.io/spring-framework/reference/web/webflux/reactive-spring.html#webflux-codecs-jackson (note at the end of section).