reuse Configuration.defaultApiClientin no-arg Api ctors and make ref volatile (fixes issue #20633)

[ronreynolds]

fixes issue #20633

this PR changes the following (all within java:native):

• fixes a broken (or at least time-zone-sensitive) unit-test from fix: CodeGen correctly produces yyyy-mm-dd format for date with examples #20516
  • also tweaks unit-test slightly so it prints value that failed expectations rather than just failing with a line number
• makes Configuration.defaultApiClient volatile so that changes will be synced across threads
• reuses Configuration.defaultApiClient in no-arg ctors of *Api classes to avoid needless creation of new ApiClients
• replaces equivalent calls to URLEncoder.encode(..., StandardCharsets.UTF_8).replaceAll("\\+", "%20") with calls to ApiClient.urlEncode(...)

PR checklist

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• Run the following to build the project

  ./mvnw clean package || exit
  

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

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  Commit all changed files.
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(is it not possible to create an alias for the java technical committee?)
@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)

[martin-mfg]

Hi @ronreynolds, thanks for the PR! The changes look good to me.
The pipeline failure seems to be unrelated to the changes in this PR. To fix the pipeline and make this PR mergeable, could you please merge the latest master into your branch? And you probably also need to update the samples, as described in the "PR checklist" in your post above.

[ronreynolds]

@martin-mfg done. sync'ed with master, generated new samples (a little shocked there were so many but i guess that's what happens when you mess with anyof, oneof, and pojo mustache files).

[wing328]

thanks for the PR. can you please review the test failure when you've time?

[ronreynolds]

@wing328 it failed due to

Run /bin/sh gradlew build -x test
Downloading https://services.gradle.org/distributions/gradle-8.7-bin.zip
............10%.............20%.............30%..
Error: The operation was canceled.

bandwidth issue? (since it appears the download started but didn't finish). 🤔

EDIT:
my bad; there was yet another issue. looking into that now.

EDIT:
looks like a missing import; not sure why it didn't show up in the local build. will push fix asap.

[ronreynolds]

pushed fix for missing import of ApiClient in 2 other model mustache files; verified build of petstore/native and petstore/native-async locally; this should address all the issues of which i am aware with this PR. 🤞

[wing328]

thanks for the PR, which has been merged