fix(go): fix Go client to normalize request header keys using Header.Set

[Wuchieh]

Use http.Header.Set when writing headerParams to http.Header in the generated Go client.

Previously, generated code directly assigned values to the underlying http.Header map:

headers[h] = []string{v}

This bypasses the header name canonicalization provided by http.Header.Set and may result in inconsistent header casing or duplicate logical headers.

This change replaces direct map assignment with:

headers.Set(h, v)

Changes

• Use http.Header.Set when writing headerParams to http.Header.
• Ensure request header names are canonicalized consistently.
• Avoid inconsistent casing or duplicate logical headers caused by direct map assignment.
• Add/update regression tests to cover the expected header handling behavior.

Related Issue

Fixes #24765

PR checklist

• Read the [contribution guidelines](https://github.com/openapitools/openapi-generator/blob/master/CONTRIBUTING.md).

• Run the following to [build the project](https://github.com/openapitools/openapi-generator#14---build-projects) and update samples:

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

• If your PR is targeting a particular programming language, @mention the relevant [technical committee](https://github.com/openapitools/openapi-generator/#62---openapi-generator-technical-committee) members.

---

Summary by cubic

Normalizes request header keys in generated Go clients by using http.Header.Set instead of direct map assignment; previously headers[h] = []string{v} wrote non-canonical names, now headers.Set(h, v) canonicalizes and de-duplicates.

• If headerParams includes case-variant duplicates, only one canonical header is sent; which value wins depends on map iteration order.
• Updated modules/openapi-generator/src/main/resources/go/client.mustache and regenerated Go sample clients to reflect the change.

^{Written for commit 8265451. Summary will update on new commits.}

[Wuchieh]

@lwj5 Please review it.

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[wing328]

please follow step 3 to update the samples so that CI can verify the change

cc @antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04) @lwj5 (2023/04)

[wing328]

please review the build failures when you've time, e.g. https://github.com/OpenAPITools/openapi-generator/actions/runs/32814668511/job/97700780542?pr=24766

[Wuchieh]

please review the build failures when you've time, e.g. https://github.com/OpenAPITools/openapi-generator/actions/runs/32814668511/job/97700780542?pr=24766

Okay, I'll take a look later to see what the issue is.