Fix HTTPS certificate/webserver not loading on first Docker start from environment variables

[pauld-0110]

Problem

When starting the DNS Server in Docker for the first time with HTTPS enabled via environment variables (e.g., DNS_SERVER_WEB_SERVICE_ENABLE_HTTPS=true), the TLS certificates failed to load, causing HTTPS to not bind.
A container restart was required for HTTPS to work properly.

Root Cause

The certificate loading process in LoadConfigFile() was called before _dnsServer was initialized in StartAsync(). The CheckAndLoadSelfSignedCertificate() method requires _dnsServer.ServerDomain to generate self-signed certificates (line 2252), which caused a NullReferenceException that was silently caught during the first startup.

Solution

Deferred certificate loading until after _dnsServer initialization by:

• Added flags to track pending certificate operations:
  _pendingLoadCustomCertificate - for custom certificate paths
  _pendingLoadSelfSignedCertificate - for self-signed certificates
• Modified LoadConfigFile() to set flags instead of immediately loading certificates when reading from environment variables
• Added certificate loading in StartAsync() after _dnsServer initialization but before web service starts

Testing

Tested on first Docker container start with:

• Self-signed certificates (DNS_SERVER_WEB_SERVICE_USE_SELF_SIGNED_CERT=true)
• Custom certificates (DNS_SERVER_WEB_SERVICE_TLS_CERTIFICATE_PATH=/path/to/cert.pfx)
• Container restart (existing config file)

Log output showing successful first-start HTTPS binding:

[2025-11-25 10:06:54 Local] Web Service TLS certificate was loaded: /etc/ssl/wildcard-certificate.pfx
[2025-11-25 10:06:54 Local] [0.0.0.0:80] [HTTP] Web Service was bound successfully.
[2025-11-25 10:06:54 Local] [0.0.0.0:443] [HTTPS] Web Service was bound successfully.

[ShreyasZare]

Thanks for the PR. Will evaluate it soon.