fix(go): fix Go client to normalize request header keys using Header.Set (#24766)

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

- Use Set when writing headerParams to http.Header to automatically apply canonical key formatting.
- Avoid inconsistent casing or duplicate header issues caused by direct map assignment.

* chore: generate samples

* chore(samples): regenerate dependency snapshots for Go and Terraform templates

- Update the go.mod Go version format for multiple Go client/server templates (e.g., 1.23 → 1.23.0, 1.25 → 1.25.0).
- Add missing terraform-plugin-testing checksums to the Terraform templates' go.sum.
- Sync checksums for indirect dependencies such as testify, x/sys, and x/text.
- Add go.sum files for templates including echo-server, chi-server, and gin-api-server-interface-only.

* chore: generate samples

* fix(test): fix abnormal test unit content.

---------

Co-authored-by: Wuchieh <wuchieh@wuchieh.com>

Author: Wuchieh

modules/openapi-generator/src/main/resources/go/client.mustache

@@ -416,7 +416,7 @@ func (c *APIClient) prepareRequest(
 	if len(headerParams) > 0 {
 		headers := http.Header{}
 		for h, v := range headerParams {
-			headers[h] = []string{v}
+			headers.Set(h, v)
 		}
 		localVarRequest.Header = headers
 	}

samples/client/echo_api/go-external-refs/client.go

@@ -29,6 +29,7 @@ github.com/hashicorp/terraform-plugin-go v0.29.0 h1:1nXKl/nSpaYIUBU1IG/EsDOX0vv+
 github.com/hashicorp/terraform-plugin-go v0.29.0/go.mod h1:vYZbIyvxyy0FWSmDHChCqKvI40cFTDGSb3D8D70i9GM=
 github.com/hashicorp/terraform-plugin-log v0.10.0 h1:eu2kW6/QBVdN4P3Ju2WiB2W3ObjkAsyfBsL3Wh1fj3g=
 github.com/hashicorp/terraform-plugin-log v0.10.0/go.mod h1:/9RR5Cv2aAbrqcTSdNmY1NRHP4E3ekrXRGjqORpXyB0=
+github.com/hashicorp/terraform-plugin-testing v1.14.0/go.mod h1:1qfWkecyYe1Do2EEOK/5/WnTyvC8wQucUkkhiGLg5nk=
 github.com/hashicorp/terraform-registry-address v0.4.0 h1:S1yCGomj30Sao4l5BMPjTGZmCNzuv7/GDTDX99E9gTk=
 github.com/hashicorp/terraform-registry-address v0.4.0/go.mod h1:LRS1Ay0+mAiRkUyltGT+UHWkIqTFvigGn/LbMshfflE=
 github.com/hashicorp/terraform-svchost v0.1.1 h1:EZZimZ1GxdqFRinZ1tpJwVxxt49xc/S52uzrw4x0jKQ=

samples/client/echo_api/go/client.go

@@ -29,6 +29,7 @@ github.com/hashicorp/terraform-plugin-go v0.29.0 h1:1nXKl/nSpaYIUBU1IG/EsDOX0vv+
 github.com/hashicorp/terraform-plugin-go v0.29.0/go.mod h1:vYZbIyvxyy0FWSmDHChCqKvI40cFTDGSb3D8D70i9GM=
 github.com/hashicorp/terraform-plugin-log v0.10.0 h1:eu2kW6/QBVdN4P3Ju2WiB2W3ObjkAsyfBsL3Wh1fj3g=
 github.com/hashicorp/terraform-plugin-log v0.10.0/go.mod h1:/9RR5Cv2aAbrqcTSdNmY1NRHP4E3ekrXRGjqORpXyB0=
+github.com/hashicorp/terraform-plugin-testing v1.14.0/go.mod h1:1qfWkecyYe1Do2EEOK/5/WnTyvC8wQucUkkhiGLg5nk=
 github.com/hashicorp/terraform-registry-address v0.4.0 h1:S1yCGomj30Sao4l5BMPjTGZmCNzuv7/GDTDX99E9gTk=
 github.com/hashicorp/terraform-registry-address v0.4.0/go.mod h1:LRS1Ay0+mAiRkUyltGT+UHWkIqTFvigGn/LbMshfflE=
 github.com/hashicorp/terraform-svchost v0.1.1 h1:EZZimZ1GxdqFRinZ1tpJwVxxt49xc/S52uzrw4x0jKQ=

samples/client/others/go/allof_multiple_ref_and_discriminator/client.go

@@ -29,6 +29,7 @@ github.com/hashicorp/terraform-plugin-go v0.29.0 h1:1nXKl/nSpaYIUBU1IG/EsDOX0vv+
 github.com/hashicorp/terraform-plugin-go v0.29.0/go.mod h1:vYZbIyvxyy0FWSmDHChCqKvI40cFTDGSb3D8D70i9GM=
 github.com/hashicorp/terraform-plugin-log v0.10.0 h1:eu2kW6/QBVdN4P3Ju2WiB2W3ObjkAsyfBsL3Wh1fj3g=
 github.com/hashicorp/terraform-plugin-log v0.10.0/go.mod h1:/9RR5Cv2aAbrqcTSdNmY1NRHP4E3ekrXRGjqORpXyB0=
+github.com/hashicorp/terraform-plugin-testing v1.14.0/go.mod h1:1qfWkecyYe1Do2EEOK/5/WnTyvC8wQucUkkhiGLg5nk=
 github.com/hashicorp/terraform-registry-address v0.4.0 h1:S1yCGomj30Sao4l5BMPjTGZmCNzuv7/GDTDX99E9gTk=
 github.com/hashicorp/terraform-registry-address v0.4.0/go.mod h1:LRS1Ay0+mAiRkUyltGT+UHWkIqTFvigGn/LbMshfflE=
 github.com/hashicorp/terraform-svchost v0.1.1 h1:EZZimZ1GxdqFRinZ1tpJwVxxt49xc/S52uzrw4x0jKQ=