[java] fix: five nondeterministic tests

[yonghanlin]

This pull request fix five non-deterministic tests that are detected by NonDex, a tool for detecting wrong assumptions on under-determined Java APIs.

Problem

• In ExampleGeneratorTest (generateFromResponseSchemaWithAllOfChildComposedModel, generateFromResponseSchemaWithAllOfComposedModel, and generateFromResponseSchemaWithDateFormat):
  The test compared the generated example against a single hard-coded JSON string. When the underlying JSON object fields were emitted in a different order, the assertion failed even though the structure and values were correct.

• JavaClientCodegenTest.testNotDuplicateOauth2FlowsScopes:
  This test assumed a fixed iteration order of getWithBasicAuthAndOauth.authMethods and getWithOauthAuth.authMethods, i.e., that index 0 would always be basic_auth and subsequent entries would always correspond to specific OAuth2 flows. Under NonDex, the backing collection order can change, causing intermittent failures.

• JetbrainsHttpClientClientCodegenTest.testBasicGenerationManyAuths:
  The test relied on a specific block layout in the generated .http file. NonDeterminism in generation (e.g., heading level changes or header ordering) caused the entire assertion to fail even when the essential content was still present.

To reproduce the failures, run NonDex on modules/openapi-generator using the following commands:

mvn -pl modules/openapi-generator edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=packageName.ClassName#methodName

Replace packageName.ClassName#methodName with one of:

org.openapitools.codegen.ExampleGeneratorTest#generateFromResponseSchemaWithAllOfChildComposedModel

org.openapitools.codegen.ExampleGeneratorTest#generateFromResponseSchemaWithAllOfComposedModel

org.openapitools.codegen.ExampleGeneratorTest#generateFromResponseSchemaWithDateFormat

org.openapitools.codegen.java.JavaClientCodegenTest#testNotDuplicateOauth2FlowsScopes

org.openapitools.codegen.jetbrains.http.client.JetbrainsHttpClientClientCodegenTest#testBasicGenerationManyAuths

The Fix

• In ExampleGeneratorTest (generateFromResponseSchemaWithAllOfChildComposedModel, generateFromResponseSchemaWithAllOfComposedModel, and generateFromResponseSchemaWithDateFormat):
  Construct an ObjectMapper and compare JsonNode by mapper.readTree(...) for both the expected JSON and examples.get(0).get("example"), so the assertion focuses on field values while ignoring non-deterministic JSON key ordering.

• JavaClientCodegenTest.testNotDuplicateOauth2FlowsScopes:
  Copy authMethods into new lists and sort them by name before performing assertions. This keeps the intent while avoiding reliance on the original collection iteration order.

• JetbrainsHttpClientClientCodegenTest.testBasicGenerationManyAuths:
  Update the assertions to check that the generated .http file contains the expected sections, request lines, and headers regardless of their order or heading level, instead of depending on a single fixed layout.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.