[MTP] testconfig.json: auto-promote scalar values for arg-bearing options

[Evangelink]

Follow-up from #6349 / #8664.

The CommandLineConfigurationSource flattens CLI options into IConfiguration with two shapes:

• Zero-arity flags → commandLineOptions:<name> = ""true"" (presence marker)
• Arg-bearing options → commandLineOptions:<name>:0, :1, … (one entry per arg)

In testconfig.json users naturally write scalars:

{
  ""commandLineOptions"": {
    ""timeout"": ""30s""
  }
}

Today this is parsed as a presence marker (commandLineOptions:timeout = ""30s""), not as the first arg of --timeout. So TryGetCommandLineOptionArguments(""timeout"") returns no args and the option silently misbehaves. The workaround documented in #8664 is to always use the array form (""timeout"": [""30s""]), but nothing enforces or guides this today.

Proposal

The new CommandLineOptionsValidator JSON pass already knows each option's registered arity. Extend it so the scalar/array shape is reconciled against arity:

1. Arg-bearing option + scalar value → auto-promote to single-element array (""timeout"": ""30s"" ⇒ ""timeout"": [""30s""]). Transparent to the user, best UX.
2. Zero-arity option + array value → fail with a clear error (""--no-banner takes no arguments"").
3. Arg-bearing option + array with wrong cardinality → already covered by the existing arity pass.

Option (1) is the high-leverage one — it removes the foot-gun entirely without breaking anything.

Alternative if we prefer strict-mode: fail-fast on scalars for arg-bearing options with an error pointing to the array form. Less friendly but unambiguous.

cc related: #8829