Add read-only data sources for all three Terraform resources

Mechanical, low-risk follow-up -- no new architectural question, no new
external service, no new write path. Each of sentry_dashboard,
sentry_alert_rule, and sentry_notification_target gets a matching data
source: a single Required id attribute in, every other attribute
Computed out, backed by the exact same getDashboard/getRule/
getNotificationTarget client methods and dashboardModelFromAPI/
alertRuleModelFromAPI/notificationTargetModelFromAPI conversion
functions the resources already use and already have tests for -- these
data sources add no new client code at all, just a thin
datasource.DataSource wrapper reusing what Create/Read/Update/Delete
already exercise.

sentry_notification_target's data source carries the same secret
caveat its resource does (Sensitive, but alerting's GET /targets/{id}
returns it unredacted, so it's a real plaintext value in Terraform
state) -- named again here rather than assumed obvious from the
resource's own docs.

Verified: provider_test.go's new schema-validation tests confirm each
data source's id is Required and everything else Computed, no
Terraform binary needed. Three new real acceptance tests
(TestAccDashboardDataSource_basic and its two siblings) each create a
resource then look it up via the matching data source, using
resource.TestCheckResourceAttrPair to prove the data source's Read
actually agrees with what the resource wrote -- not just that both
compile. Skip-gated by TF_ACC same as the existing six acceptance
tests, and needs the same live api/alerting services this environment
has no Docker access to bring up, so not run here -- same disclosed gap
as everything else Docker-gated in this repo.

terraform/internal/provider/provider_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"testing"
 
+	"github.com/hashicorp/terraform-plugin-framework/datasource"
 	"github.com/hashicorp/terraform-plugin-framework/provider"
 	"github.com/hashicorp/terraform-plugin-framework/resource"
 )
@@ -133,3 +134,75 @@ func TestNotificationTargetResourceMetadataSetsTypeName(t *testing.T) {
 		t.Fatalf("TypeName = %q, want sentry_notification_target", resp.TypeName)
 	}
 }
+
+func TestDashboardDataSourceSchemaValid(t *testing.T) {
+	ctx := context.Background()
+	req := datasource.SchemaRequest{}
+	resp := &datasource.SchemaResponse{}
+
+	newDashboardDataSource().Schema(ctx, req, resp)
+
+	if resp.Diagnostics.HasError() {
+		t.Fatalf("sentry_dashboard data source schema has errors: %v", resp.Diagnostics)
+	}
+	if !resp.Schema.Attributes["id"].IsRequired() {
+		t.Error(`"id" must be Required -- a data source needs it to know what to look up`)
+	}
+	if !resp.Schema.Attributes["name"].IsComputed() {
+		t.Error(`"name" must be Computed`)
+	}
+}
+
+func TestAlertRuleDataSourceSchemaValid(t *testing.T) {
+	ctx := context.Background()
+	req := datasource.SchemaRequest{}
+	resp := &datasource.SchemaResponse{}
+
+	newAlertRuleDataSource().Schema(ctx, req, resp)
+
+	if resp.Diagnostics.HasError() {
+		t.Fatalf("sentry_alert_rule data source schema has errors: %v", resp.Diagnostics)
+	}
+	if !resp.Schema.Attributes["id"].IsRequired() {
+		t.Error(`"id" must be Required`)
+	}
+	if !resp.Schema.Attributes["query"].IsComputed() {
+		t.Error(`"query" must be Computed`)
+	}
+}
+
+func TestNotificationTargetDataSourceSchemaValid(t *testing.T) {
+	ctx := context.Background()
+	req := datasource.SchemaRequest{}
+	resp := &datasource.SchemaResponse{}
+
+	newNotificationTargetDataSource().Schema(ctx, req, resp)
+
+	if resp.Diagnostics.HasError() {
+		t.Fatalf("sentry_notification_target data source schema has errors: %v", resp.Diagnostics)
+	}
+	if !resp.Schema.Attributes["id"].IsRequired() {
+		t.Error(`"id" must be Required`)
+	}
+	if !resp.Schema.Attributes["secret"].IsSensitive() {
+		t.Error(`"secret" must be Sensitive`)
+	}
+}
+
+func TestDataSourcesMetadataSetTypeNames(t *testing.T) {
+	cases := []struct {
+		newDS    func() datasource.DataSource
+		wantType string
+	}{
+		{newDashboardDataSource, "sentry_dashboard"},
+		{newAlertRuleDataSource, "sentry_alert_rule"},
+		{newNotificationTargetDataSource, "sentry_notification_target"},
+	}
+	for _, c := range cases {
+		resp := &datasource.MetadataResponse{}
+		c.newDS().Metadata(context.Background(), datasource.MetadataRequest{ProviderTypeName: "sentry"}, resp)
+		if resp.TypeName != c.wantType {
+			t.Errorf("TypeName = %q, want %q", resp.TypeName, c.wantType)
+		}
+	}
+}