integration: add SSH check mode tests

Add ReadLog method to headscale integration container for log
inspection. Split SSH check mode tests into CLI and OIDC variants
and add comprehensive test coverage:

- TestSSHOneUserToOneCheckModeCLI: basic check mode with CLI approval
- TestSSHOneUserToOneCheckModeOIDC: check mode with OIDC approval
- TestSSHCheckModeUnapprovedTimeout: rejection on cache expiry
- TestSSHCheckModeCheckPeriodCLI: session expiry and re-auth
- TestSSHCheckModeAutoApprove: auto-approval within check period
- TestSSHCheckModeNegativeCLI: explicit rejection via CLI

Update existing integration tests to use headscale auth register.

Updates #1850

integration/control.go

@@ -16,6 +16,7 @@ import (
 type ControlServer interface {
 	Shutdown() (string, string, error)
 	SaveLog(path string) (string, string, error)
+	ReadLog() (string, string, error)
 	SaveProfile(path string) error
 	Execute(command []string) (string, error)
 	WriteFile(path string, content []byte) error