[FEATURE] Add enhanced logging for TLS handshake failures in mutual_tls authentication

Is your feature request related to a problem? Please describe.

When mutual_tls authentication is enabled (#6916), operators cannot easily distinguish
TLS handshake failures caused by client certificate rejections from network-level connection
issues. Armeria logs handshake failures at WARN level with minimal context.

Describe the solution you'd like

Add structured logging for TLS handshake failures when mutual_tls is configured, including:

• Client IP address
• Failure reason (e.g., "certificate not trusted", "no client certificate presented",
  "certificate expired")
• Subject DN of the rejected certificate (if available)

This would allow operators to quickly identify:

• Clients connecting without certificates
• Clients using expired certificates
• Clients signed by an untrusted CA

Implementation could use Armeria's ServerBuilder.childChannelPipelineCustomizer() to add
a Netty handler that listens for SslHandshakeCompletionEvent failures and emits a
structured log with the relevant details.

Additional context

Identified during review of #6916. Armeria's default WARN logging provides basic visibility
but does not include enough context to diagnose client-side misconfigurations.