Validate mirth.properties at startup and refuse to run on any invalid value #413

mirth.properties values are read ad-hoc across the codebase with inline defaults, and validation is inconsistent: some settings fail fast, some crash, some silently coerce. On adjacent lines of Mirth.run(), http.port = banana throws an uncaught ConversionException via isUsingHttp()'s default-less getInt and the server thread dies with a stack trace, while https.port = banana right next to it refuses to start cleanly with "port is invalid" (testPort). Booleans coerce to false (Boolean.parseBoolean), https.*.protocols passes through unvalidated, while a few settings do have ad-hoc guards (the DH key-size clamp, password requirements). Same class of mistake, different outcomes.

Target the model nginx (nginx -t), Apache (configtest), haproxy (-c), and sshd (-t) use: one validation pass over the whole merged config (base + conf/mirth.properties.d drop-ins, #405) at startup, refuse to run on any invalid value, with a clear message naming the property and the reason. Never start on a config that doesn't fully parse.

Why this is architectural, not a patch:

• No config schema exists. nginx validates everything because its parser is the schema; every directive has a known type and constraints. mirth.properties has no central registry of valid key -> type -> constraints -> required. Step one is building that registry.
• Validate the merged config against the registry at startup; fail closed with a per-property message on any type/range/enum violation.
• Mistyped keys can't be caught the way nginx catches an unknown directive: htp.port = 8080 silently leaves http.port at its default. The registry would need to flag unknown keys (warn or fail) - a design call.
• Offer a check mode (-t-style "validate and exit") so operators can test before a restart.

Context: came up during #405, which added fail-closed handling for unreadable/malformed drop-in files. This covers invalid values in readable files.