Fix race conditions and improve lifecycle safety (#150)

## Summary
Comprehensive fixes for race conditions, concurrency bugs, and code quality improvements in the lifecycle module and related test files.

## Changes

### Race Condition Fixes
- Fixed data race in `SetChannel()` and `Channel()` by adding mutex protection
- Fixed data race in `StartedAt()` by adding mutex protection
- Fixed test race conditions in `transport` and `interaction` packages where variables were shared between goroutines

### Concurrency Safety Improvements
- Refactored `Close()` to release mutex before calling external code, preventing potential deadlocks
- Made `SetChannel()` reject calls after `Close()` to prevent setting channel on torn-down lifecycle
- Made `SetStatus()` ignore calls after `Close()` to ensure CLOSED is a terminal state
- Added nil check to `SetChannel()` to prevent masking upstream bugs

### Code Quality Improvements
- Refactored `Close()` to extract helper methods: `closeChannel()`, `closeConnection()`, `cleanupRegistry()`, `cleanupForwarder()`
- Fixed `isClosedError()` to remove redundant string comparison
- Added `PortRegistry` interface for better interface segregation (removed dependency on full `port.Port`)
- Made `cleanupRegistry()` conditional on non-empty slug to avoid unnecessary registry calls
- Fixed `startedAt` to be set when session starts running instead of at object creation

Reviewed-on: #150
Co-authored-by: Bagas <bagas@fossy.my.id>
Co-committed-by: Bagas <bagas@fossy.my.id>

internal/registry/registry_test.go

@@ -4,7 +4,6 @@ import (
 	"sync"
 	"testing"
 	"time"
-	"tunnel_pls/internal/port"
 	"tunnel_pls/internal/session/forwarder"
 	"tunnel_pls/internal/session/interaction"
 	"tunnel_pls/internal/session/lifecycle"
@@ -78,15 +77,15 @@ func (ml *mockLifecycle) Connection() ssh.Conn {
 	return args.Get(0).(ssh.Conn)
 }
 
-func (ml *mockLifecycle) PortRegistry() port.Port {
+func (ml *mockLifecycle) PortRegistry() lifecycle.PortRegistry {
 	args := ml.Called()
 	if args.Get(0) == nil {
 		return nil
 	}
-	return args.Get(0).(port.Port)
+	return args.Get(0).(lifecycle.PortRegistry)
 }
 
-func (ml *mockLifecycle) SetChannel(channel ssh.Channel)       { ml.Called(channel) }
+func (ml *mockLifecycle) SetChannel(channel ssh.Channel) error { return ml.Called(channel).Error(0) }
 func (ml *mockLifecycle) SetStatus(status types.SessionStatus) { ml.Called(status) }
 func (ml *mockLifecycle) IsActive() bool                       { return ml.Called().Bool(0) }
 func (ml *mockLifecycle) StartedAt() time.Time                 { return ml.Called().Get(0).(time.Time) }