integration: standardize test infrastructure options

Make embedded DERP server and TLS the default configuration for all
integration tests, replacing the per-test opt-in model that led to
inconsistent and flaky test behavior.

Infrastructure changes:
- DefaultConfigEnv() includes embedded DERP server settings
- New() auto-generates a proper CA + server TLS certificate pair
- CA cert is installed into container trust stores and returned by
  GetCert() so clients and internal tools (curl) trust the server
- CreateCertificate() now returns (caCert, cert, key) instead of
  discarding the CA certificate
- Add WithPublicDERP() and WithoutTLS() opt-out options
- Remove WithTLS(), WithEmbeddedDERPServerOnly(), and WithDERPAsIP()
  since all their behavior is now the default or unnecessary

Test cleanup:
- Remove all redundant WithTLS/WithEmbeddedDERPServerOnly/WithDERPAsIP
  calls from test files
- Give every test a unique WithTestName by parameterizing aclScenario,
  sshScenario, and derpServerScenario helpers
- Add WithTestName to tests that were missing it
- Document all non-standard options with inline comments explaining
  why each is needed

Updates #3139

integration/dns_test.go

@@ -104,8 +104,6 @@ func TestResolveMagicDNSExtraRecordsPath(t *testing.T) {
 			"HEADSCALE_DNS_EXTRA_RECORDS_PATH": erPath,
 		}),
 		hsic.WithFileInContainer(erPath, b),
-		hsic.WithEmbeddedDERPServerOnly(),
-		hsic.WithTLS(),
 	)
 	requireNoErrHeadscaleEnv(t, err)