test(linear): add comprehensive tests for mapping and client functions

Added tests for:
- DefaultMappingConfig
- PriorityToBeads/PriorityToLinear
- StateToBeadsStatus/ParseBeadsStatus
- StatusToLinearStateType
- LabelToIssueType/ParseIssueType
- RelationToBeadsDep
- IssueToBeads (with parent/child handling)
- BuildLinearToLocalUpdates
- LoadMappingConfig
- BuildLinearDescription
- NewClient/WithEndpoint/WithHTTPClient
- ExtractLinearIdentifier
- IsLinearExternalRef

Linear package coverage improved from 14.3% to ~50%.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/linear/client_test.go

@@ -1,6 +1,10 @@
 package linear
 
-import "testing"
+import (
+	"net/http"
+	"testing"
+	"time"
+)
 
 func TestCanonicalizeLinearExternalRef(t *testing.T) {
 	tests := []struct {
@@ -39,3 +43,130 @@ func TestCanonicalizeLinearExternalRef(t *testing.T) {
 		}
 	}
 }
+
+func TestNewClient(t *testing.T) {
+	client := NewClient("test-api-key", "test-team-id")
+
+	if client.APIKey != "test-api-key" {
+		t.Errorf("APIKey = %q, want %q", client.APIKey, "test-api-key")
+	}
+	if client.TeamID != "test-team-id" {
+		t.Errorf("TeamID = %q, want %q", client.TeamID, "test-team-id")
+	}
+	if client.Endpoint != DefaultAPIEndpoint {
+		t.Errorf("Endpoint = %q, want %q", client.Endpoint, DefaultAPIEndpoint)
+	}
+	if client.HTTPClient == nil {
+		t.Error("HTTPClient should not be nil")
+	}
+}
+
+func TestWithEndpoint(t *testing.T) {
+	client := NewClient("key", "team")
+	customEndpoint := "https://custom.linear.app/graphql"
+
+	newClient := client.WithEndpoint(customEndpoint)
+
+	if newClient.Endpoint != customEndpoint {
+		t.Errorf("Endpoint = %q, want %q", newClient.Endpoint, customEndpoint)
+	}
+	// Original should be unchanged
+	if client.Endpoint != DefaultAPIEndpoint {
+		t.Errorf("Original endpoint changed: %q", client.Endpoint)
+	}
+	// Other fields preserved
+	if newClient.APIKey != "key" {
+		t.Errorf("APIKey not preserved: %q", newClient.APIKey)
+	}
+}
+
+func TestWithHTTPClient(t *testing.T) {
+	client := NewClient("key", "team")
+	customHTTPClient := &http.Client{Timeout: 60 * time.Second}
+
+	newClient := client.WithHTTPClient(customHTTPClient)
+
+	if newClient.HTTPClient != customHTTPClient {
+		t.Error("HTTPClient not set correctly")
+	}
+	// Other fields preserved
+	if newClient.APIKey != "key" {
+		t.Errorf("APIKey not preserved: %q", newClient.APIKey)
+	}
+	if newClient.Endpoint != DefaultAPIEndpoint {
+		t.Errorf("Endpoint not preserved: %q", newClient.Endpoint)
+	}
+}
+
+func TestExtractLinearIdentifier(t *testing.T) {
+	tests := []struct {
+		name        string
+		externalRef string
+		want        string
+	}{
+		{
+			name:        "standard URL",
+			externalRef: "https://linear.app/team/issue/PROJ-123",
+			want:        "PROJ-123",
+		},
+		{
+			name:        "URL with slug",
+			externalRef: "https://linear.app/team/issue/PROJ-456/some-title-here",
+			want:        "PROJ-456",
+		},
+		{
+			name:        "URL with trailing slash",
+			externalRef: "https://linear.app/team/issue/ABC-789/",
+			want:        "ABC-789",
+		},
+		{
+			name:        "non-linear URL",
+			externalRef: "https://jira.example.com/browse/PROJ-123",
+			want:        "",
+		},
+		{
+			name:        "empty string",
+			externalRef: "",
+			want:        "",
+		},
+		{
+			name:        "malformed URL",
+			externalRef: "not-a-url",
+			want:        "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ExtractLinearIdentifier(tt.externalRef)
+			if got != tt.want {
+				t.Errorf("ExtractLinearIdentifier(%q) = %q, want %q", tt.externalRef, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestIsLinearExternalRef(t *testing.T) {
+	tests := []struct {
+		ref  string
+		want bool
+	}{
+		{"https://linear.app/team/issue/PROJ-123", true},
+		{"https://linear.app/team/issue/PROJ-123/slug", true},
+		{"https://jira.example.com/browse/PROJ-123", false},
+		{"https://github.com/org/repo/issues/123", false},
+		{"", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.ref, func(t *testing.T) {
+			got := IsLinearExternalRef(tt.ref)
+			if got != tt.want {
+				t.Errorf("IsLinearExternalRef(%q) = %v, want %v", tt.ref, got, tt.want)
+			}
+		})
+	}
+}
+
+// Note: BuildStateCache and FindStateForBeadsStatus require API calls
+// and would need mocking to test. Skipping unit tests for those.