refactor: co-locate config, container, envpool, errors, experiment, jupyter tests

Move unit tests from tests/unit/ to internal/ following Go conventions:
- tests/unit/config/* -> internal/config/* (constants, mode_paths, paths, validation)
- tests/unit/container/* -> internal/container/* (podman, security tests)
- tests/unit/envpool/* -> internal/envpool/* (envpool tests)
- tests/unit/errors/* -> internal/errtypes/* (errors_test.go moved to errtypes package)
- tests/unit/experiment/* -> internal/experiment/* (manager tests)
- tests/unit/jupyter/* -> internal/jupyter/* (config, package_blacklist, service_manager, trash_restore)

Update import paths in test files to reflect new locations.

Note: errors_test.go moved from tests/unit/errors/ to internal/errtypes/ to match the package structure.

internal/config/constants_test.go

@@ -1,4 +1,4 @@
-package config
+package config_test
 
 import (
 	"testing"

internal/config/mode_paths_test.go

@@ -1,4 +1,4 @@
-package config
+package config_test
 
 import (
 	"testing"

internal/config/paths_test.go

@@ -1,4 +1,4 @@
-package config
+package config_test
 
 import (
 	"os"

internal/config/validation_test.go

@@ -1,4 +1,4 @@
-package config
+package config_test
 
 import (
 	"fmt"

internal/container/podman_test.go

@@ -1,4 +1,4 @@
-package tests
+package container_test
 
 import (
 	"context"

internal/container/security_test.go

@@ -1,4 +1,4 @@
-package tests
+package container_test
 
 import (
 	"errors"

internal/errtypes/errors_test.go

@@ -1,4 +1,4 @@
-package tests
+package errtypes_test
 
 import (
 	"errors"

internal/experiment/manager_test.go

@@ -1,4 +1,4 @@
-package experiment
+package experiment_test
 
 import (
 	"os"