fix(telemetry): warn instead of crashing when OTLP endpoint has no enabled exporters (#4648)

* fix(telemetry): warn instead of crashing when OTLP endpoint has no enabled exporters

When an OTLP endpoint is configured via `thv config otel set-endpoint`
but both tracing and metrics are disabled, `thv serve` crashes with a
fatal validation error preventing the server from starting.

Instead of failing, log a warning and clear the endpoint so the server
starts normally. This avoids a confusing "Health check failed" error
in ToolHive Studio.

Fixes #4647

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor(telemetry): extract handleUnusedEndpoint to reduce cyclomatic complexity

Move the unused endpoint check into a dedicated helper function to
keep NewServeProvider under the gocyclo threshold (15).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(telemetry): enable tracing by default when OTLP endpoint has no enabled exporters

Instead of clearing the endpoint (which would silently drop telemetry
for users who configured an endpoint expecting it to work), enable
tracing by default so the endpoint is actually used.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: peppescg

pkg/telemetry/serve.go

@@ -29,6 +29,9 @@ func NewServeProvider(ctx context.Context) (provider *Provider, otelEnabled bool
 
 	otelCfg := appConfig.OTEL
 	hasRegisteredProcessors := HasRegisteredSpanProcessors()
+
+	handleUnusedEndpoint(&otelCfg)
+
 	if otelCfg.Endpoint == "" && !otelCfg.EnablePrometheusMetricsPath && !hasRegisteredProcessors {
 		return nil, false, nil
 	}
@@ -74,3 +77,20 @@ func NewServeProvider(ctx context.Context) (provider *Provider, otelEnabled bool
 
 	return p, true, nil
 }
+
+// handleUnusedEndpoint enables tracing by default when an OTLP endpoint is
+// configured but both tracing and metrics are disabled, so the server can start
+// normally instead of crashing with a fatal validation error.
+func handleUnusedEndpoint(otelCfg *config.OpenTelemetryConfig) {
+	if otelCfg.Endpoint == "" {
+		return
+	}
+	tracingOff := otelCfg.TracingEnabled == nil || !*otelCfg.TracingEnabled
+	metricsOff := otelCfg.MetricsEnabled == nil || !*otelCfg.MetricsEnabled
+	if tracingOff && metricsOff {
+		slog.Warn("OTLP endpoint is configured but tracing and metrics are both disabled; enabling tracing by default",
+			"endpoint", otelCfg.Endpoint)
+		enabled := true
+		otelCfg.TracingEnabled = &enabled
+	}
+}