Fix API validation inconsistency: Add required validation to AssignRoleToAPIUserRequest.name

During RevokeRoleFromAPIUser implementation, discovered that AssignRoleToAPIUserRequest
was missing `required: true` validation on the name field, while RevokeRoleFromAPIUserRequest
correctly included it.

Both methods are mirror operations on API user resources and must have symmetric validation:
- Both require a valid API user resource name to function
- Neither can operate with empty/null name values
- Schema-level validation provides fail-fast behavior

This fix ensures API consistency before publication, avoiding future breaking changes.

Changes:
- Added `required: true` to AssignRoleToAPIUserRequest.name field validation
- Regenerated Go SDK with updated validation rules

Validation:
- buf lint: PASSED
- go build: PASSED
- ruff check: PASSED

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: BRBussy

go/iam/api_user/v1/service.pb.go

@@ -216,6 +216,7 @@ message AssignRoleToAPIUserRequest {
      Name of the api user to assign a role to.
   */
   string name = 1 [(buf.validate.field) = {
+    required: true,
     string: {
       len: 36,
       pattern: "^api_users/[0123456789ABCDEFGHJKMNPQRSTVWXYZ]{26}$"