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. This ensures canonical casing and prevents duplicate logical headers.

• Old behavior: headers[h] = []string{v} wrote headers without canonicalizing names. New behavior: headers.Set(h, v) canonicalizes and de-duplicates.
• If headerParams contains case-variant duplicates, only one canonical header is sent; the winning value depends on map iteration order.
• Regenerate clients to apply the change; the update is in modules/openapi-generator/src/main/resources/go/client.mustache.

^{Written for commit 70b0c1e. Summary will update on new commits.}

[Wuchieh]

@lwj5 Please review it.

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}