fix(copilot): use dynamic API limits to prevent prompt token overflow

The Copilot API enforces per-account prompt token limits (128K individual,
168K business) that differ from the static 200K context length advertised
by the proxy. This mismatch caused Claude Code to accumulate context
beyond the actual limit, triggering "prompt token count exceeds the limit
of 128000" errors.

Changes:
- Extract max_prompt_tokens and max_output_tokens from the Copilot
  /models API response (capabilities.limits) and use them as the
  authoritative ContextLength and MaxCompletionTokens values
- Add CopilotModelLimits struct and Limits() helper to parse limits
  from the existing Capabilities map
- Fix GitLab Duo context-1m beta header not being set when routing
  through the Anthropic gateway (gitlab_duo_force_context_1m attr
  was set but only gin headers were checked)
- Fix flaky parallel tests that shared global model registry state

internal/runtime/executor/github_copilot_executor_test.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 	"testing"
 
+	copilotauth "github.com/router-for-me/CLIProxyAPI/v6/internal/auth/copilot"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/registry"
 	cliproxyexecutor "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
 	sdktranslator "github.com/router-for-me/CLIProxyAPI/v6/sdk/translator"
@@ -74,7 +75,7 @@ func TestUseGitHubCopilotResponsesEndpoint_CodexModel(t *testing.T) {
 }
 
 func TestUseGitHubCopilotResponsesEndpoint_RegistryResponsesOnlyModel(t *testing.T) {
-	t.Parallel()
+	// Not parallel: shares global model registry with DynamicRegistryWinsOverStatic.
 	if !useGitHubCopilotResponsesEndpoint(sdktranslator.FromString("openai"), "gpt-5.4") {
 		t.Fatal("expected responses-only registry model to use /responses")
 	}
@@ -84,7 +85,7 @@ func TestUseGitHubCopilotResponsesEndpoint_RegistryResponsesOnlyModel(t *testing
 }
 
 func TestUseGitHubCopilotResponsesEndpoint_DynamicRegistryWinsOverStatic(t *testing.T) {
-	t.Parallel()
+	// Not parallel: mutates global model registry, conflicts with RegistryResponsesOnlyModel.
 
 	reg := registry.GetGlobalRegistry()
 	clientID := "github-copilot-test-client"
@@ -706,3 +707,111 @@ func TestStripUnsupportedBetas_AllBetasStripped(t *testing.T) {
 		t.Fatal("betas field should be deleted when all betas are stripped")
 	}
 }
+
+func TestCopilotModelEntry_Limits(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		capabilities map[string]any
+		wantNil      bool
+		wantPrompt   int
+		wantOutput   int
+		wantContext  int
+	}{
+		{
+			name:         "nil capabilities",
+			capabilities: nil,
+			wantNil:      true,
+		},
+		{
+			name:         "no limits key",
+			capabilities: map[string]any{"family": "claude-opus-4.6"},
+			wantNil:      true,
+		},
+		{
+			name:         "limits is not a map",
+			capabilities: map[string]any{"limits": "invalid"},
+			wantNil:      true,
+		},
+		{
+			name: "all zero values",
+			capabilities: map[string]any{
+				"limits": map[string]any{
+					"max_context_window_tokens": float64(0),
+					"max_prompt_tokens":         float64(0),
+					"max_output_tokens":         float64(0),
+				},
+			},
+			wantNil: true,
+		},
+		{
+			name: "individual account limits (128K prompt)",
+			capabilities: map[string]any{
+				"limits": map[string]any{
+					"max_context_window_tokens": float64(144000),
+					"max_prompt_tokens":         float64(128000),
+					"max_output_tokens":         float64(64000),
+				},
+			},
+			wantNil:     false,
+			wantPrompt:  128000,
+			wantOutput:  64000,
+			wantContext: 144000,
+		},
+		{
+			name: "business account limits (168K prompt)",
+			capabilities: map[string]any{
+				"limits": map[string]any{
+					"max_context_window_tokens": float64(200000),
+					"max_prompt_tokens":         float64(168000),
+					"max_output_tokens":         float64(32000),
+				},
+			},
+			wantNil:     false,
+			wantPrompt:  168000,
+			wantOutput:  32000,
+			wantContext: 200000,
+		},
+		{
+			name: "partial limits (only prompt)",
+			capabilities: map[string]any{
+				"limits": map[string]any{
+					"max_prompt_tokens": float64(128000),
+				},
+			},
+			wantNil:    false,
+			wantPrompt: 128000,
+			wantOutput: 0,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			entry := copilotauth.CopilotModelEntry{
+				ID:           "claude-opus-4.6",
+				Capabilities: tt.capabilities,
+			}
+			limits := entry.Limits()
+			if tt.wantNil {
+				if limits != nil {
+					t.Fatalf("expected nil limits, got %+v", limits)
+				}
+				return
+			}
+			if limits == nil {
+				t.Fatal("expected non-nil limits, got nil")
+			}
+			if limits.MaxPromptTokens != tt.wantPrompt {
+				t.Errorf("MaxPromptTokens = %d, want %d", limits.MaxPromptTokens, tt.wantPrompt)
+			}
+			if limits.MaxOutputTokens != tt.wantOutput {
+				t.Errorf("MaxOutputTokens = %d, want %d", limits.MaxOutputTokens, tt.wantOutput)
+			}
+			if tt.wantContext > 0 && limits.MaxContextWindowTokens != tt.wantContext {
+				t.Errorf("MaxContextWindowTokens = %d, want %d", limits.MaxContextWindowTokens, tt.wantContext)
+			}
+		})
+	}
+}