fix(security): enable system trusted root certificates in buildSecurityContext

[ZhuchkaTriplesix]

Summary

In buildSecurityContext (lib/core/security/ssl_certificate_support.dart, lines 89–102):

SecurityContext? buildSecurityContext(SslCertificatePaths paths) {
  if (!paths.hasAny) return null;
  final context = SecurityContext();
  if (SslCertificatePaths._nonEmpty(paths.clientCert)) {
    context.useCertificateChain(paths.clientCert!.trim());
  }
  if (SslCertificatePaths._nonEmpty(paths.clientKey)) {
    context.usePrivateKey(paths.clientKey!.trim());
  }
  if (SslCertificatePaths._nonEmpty(paths.rootCert)) {
    context.setTrustedCertificates(paths.rootCert!.trim());
  }
  return context;
}

In Dart dart:io, SecurityContext() defaults to withTrustedRoots: false.

When a user configures client certificate authentication (mTLS) with clientCert and clientKey, but does not provide a custom rootCert (expecting the server certificate to be validated against the operating system's trusted root CAs, such as for AWS RDS, Google Cloud SQL, or Supabase):
The resulting SecurityContext has zero trusted root certificates. The TLS handshake fails immediately with:
HandshakeException: Certificate verify failed (unable to get local issuer certificate).

Repro

1. Configure a PostgreSQL, MySQL, or Redis connection with clientCert and clientKey, leaving Root CA empty.
2. Attempt to connect to a cloud database with a valid public TLS certificate.
3. Observe handshake failure due to missing trusted root certificates.

Scope

• Initialize SecurityContext(withTrustedRoots: true) in buildSecurityContext so that system root CAs are available when no custom root certificate is provided.
• If a custom rootCert is provided, add it via context.setTrustedCertificates().

Out of scope

• Insecure skip-verification flags (badCertificateCallback).

[ZhuchkaTriplesix]

Resolved by #948 (already merged into dev; not auto-closed since this repo's default branch is main and PRs merge into dev).