fix(auth): strip model suffix in GitHub Copilot executor before upstream call

GitHub Copilot API rejects model names with suffixes (e.g. claude-opus-4.6(medium)).
The OAuthModelAlias resolution correctly maps aliases like 'opus(medium)' to
'claude-opus-4.6(medium)' preserving the suffix, but the executor must strip the
suffix before sending to the upstream API since Copilot only accepts bare model names.

Update normalizeModel in github_copilot_executor to strip suffixes using
thinking.ParseSuffix, matching the pattern used by other executors.

Also add test coverage for:
- OAuthModelAliasChannel github-copilot and kiro channel resolution
- Suffix preservation in alias resolution for github-copilot
- normalizeModel suffix stripping in github_copilot_executor

internal/runtime/executor/github_copilot_executor_test.go

@@ -0,0 +1,54 @@
+package executor
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestGitHubCopilotNormalizeModel_StripsSuffix(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name      string
+		model     string
+		wantModel string
+	}{
+		{
+			name:      "suffix stripped",
+			model:     "claude-opus-4.6(medium)",
+			wantModel: "claude-opus-4.6",
+		},
+		{
+			name:      "no suffix unchanged",
+			model:     "claude-opus-4.6",
+			wantModel: "claude-opus-4.6",
+		},
+		{
+			name:      "different suffix stripped",
+			model:     "gpt-4o(high)",
+			wantModel: "gpt-4o",
+		},
+		{
+			name:      "numeric suffix stripped",
+			model:     "gemini-2.5-pro(8192)",
+			wantModel: "gemini-2.5-pro",
+		},
+	}
+
+	e := &GitHubCopilotExecutor{}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			body := []byte(`{"model":"` + tt.model + `","messages":[]}`)
+			got := e.normalizeModel(tt.model, body)
+
+			gotModel := gjson.GetBytes(got, "model").String()
+			if gotModel != tt.wantModel {
+				t.Fatalf("normalizeModel() model = %q, want %q", gotModel, tt.wantModel)
+			}
+		})
+	}
+}