Address PR review comments for PECOBLR-1147

Changes:
- Add thread safety to telemetryClient start() and close() methods with mutex
- Add debug logging when refCount becomes negative to detect bugs
- Verified no race conditions with `go test -race`

Review feedback addressed:
1. Made client methods thread-safe for future background operations
2. Added logging to catch potential reference counting bugs
3. Confirmed automatic cleanup via refCount is sufficient (no LRU needed)

All tests pass including race detector.

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

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

Author: samikshya-db

telemetry/client.go

@@ -2,6 +2,7 @@ package telemetry
 
 import (
 	"net/http"
+	"sync"
 )
 
 // telemetryClient represents a client for sending telemetry data to Databricks.
@@ -10,6 +11,7 @@ type telemetryClient struct {
 	host       string
 	httpClient *http.Client
 	cfg        *Config
+	mu         sync.Mutex
 	started    bool
 	closed     bool
 }
@@ -26,13 +28,17 @@ func newTelemetryClient(host string, httpClient *http.Client, cfg *Config) *tele
 // start starts the telemetry client's background operations.
 // This is a stub implementation that will be fully implemented in Phase 4.
 func (c *telemetryClient) start() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
 	c.started = true
 	return nil
 }
 
 // close stops the telemetry client and flushes any pending data.
 // This is a stub implementation that will be fully implemented in Phase 4.
 func (c *telemetryClient) close() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
 	c.closed = true
 	return nil
 }

telemetry/manager.go

@@ -3,6 +3,8 @@ package telemetry
 import (
 	"net/http"
 	"sync"
+
+	"github.com/databricks/databricks-sql-go/logger"
 )
 
 // clientManager manages one telemetry client per host.
@@ -62,6 +64,10 @@ func (m *clientManager) releaseClient(host string) error {
 	}
 
 	holder.refCount--
+	if holder.refCount < 0 {
+		// This should never happen - indicates a bug where releaseClient was called more than getOrCreateClient
+		logger.Logger.Debug().Str("host", host).Int("refCount", holder.refCount).Msg("telemetry client refCount became negative")
+	}
 	if holder.refCount <= 0 {
 		delete(m.clients, host)
 		m.mu.Unlock()