fix(postgres): only enable SSL when cert and key files actually exist on disk

[burnerlee]

PR checklist:

• Did you run ClangFormat ? Yes (no formatting changes in modified lines)
• Did you separate headers to a different section in existing community code base ? N/A (no new headers in inherited code)
• Did you surround proton: starts/ends for new code in existing community code base ? N/A (PostgreSQLHandlerFactory is not ClickHouse-inherited code)

Please write user-readable short description of the changes:

Problem

Connecting to Proton's PostgreSQL port (5432) with psql fails immediately:

psql: error: connection to server at "127.0.0.1", port 5432 failed: SSL SYSCALL error: EOF detected
        connection to server at "127.0.0.1", port 5432 failed: server closed the connection unexpectedly

This happens on any deployment where SSL certificate files are not present (the default).

Root Cause

PostgreSQLHandlerFactory hardcoded ssl_enabled = true at compile time whenever USE_SSL was set. The psql client always sends an SSL_REQUEST first before the startup message. The handler entered makeSecureConnectionSSL(), which called Poco::Net::SSLManager::instance().defaultServerContext() — this throws/fails when no cert/key files are configured, and the connection drops with EOF before any error can be sent back to the client.

Fix

Default ssl_enabled to false. In the factory constructor, read openSSL.server.certificateFile and openSSL.server.privateKeyFile from the server config at runtime. Only set ssl_enabled = true when both keys are non-empty and both files exist on disk.

When ssl_enabled = false, the handler correctly responds 'N' to SSL_REQUEST, and psql falls back to plaintext — the expected behavior for a server without SSL configured.

Fixes #824

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