[julia-server] Use req.headers for HTTP.jl 2.x compatibility

[tanmaykm]

HTTP.jl 2.x removed the single-arg HTTP.headers(req) accessor, so generated server read-handlers with header params throw a MethodError (500) under HTTP 2. Use req.headers instead, which works on both HTTP.jl 1.x and 2.x.

Regenerated the julia-server petstore sample to match.

PR checklist

• Read the contribution guidelines.
• 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.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Switch generated Julia server header parsing to req.headers to restore compatibility with HTTP.jl 2.x and prevent 500 MethodErrors when handlers read header params. Remains compatible with HTTP.jl 1.x; regenerated julia-server petstore samples to match.

^{Written for commit 5abb026. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}