update rust axum samples, add cargo fmt

[wing328]

a follow up PR to https://github.com/OpenAPITools/openapi-generator/pull/18895

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

  (For Windows users, please run the script in Git BASH) 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 7.6.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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[wing328]

tests passed locally:

test output\rust-axum-test\src\types.rs - types::Nullable<T>::unwrap_or_default (line 562) ... ok
test output\rust-axum-test\src\types.rs - types::Nullable<T>::unwrap_or_else (line 249) ... ok

test result: ok. 22 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.92s

[INFO]
[INFO] --- exec:1.2.1:exec (clippy) @ RustServerTests ---
    Finished dev [unoptimized + debuginfo] target(s) in 0.26s
[INFO]
[INFO] --- exec:1.2.1:exec (fmt) @ RustServerTests ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  30.441 s
[INFO] Finished at: 2024-06-15T18:30:52+08:00
[INFO] ------------------------------------------------------------------------
[INFO] 5 goals, 5 executed
[INFO]
[INFO] Publishing build scan...
[INFO] https://ge.openapi-generator.tech/s/ri3mhv2p5l7nw