[Enhancement]: Configuration validator only validates layers, not mission-level fields

The validate() function in API/Backend/Config/validate.js only checks that msv, layers, and tools exist at the top level, then validates layer-specific fields (URLs, zooms, model params, etc.). It does not validate msv contents (mission name, view, radius), projection, look, time, panels, coordinates, or tools array contents. Invalid mission-level configuration can be saved via the API without any warning.

Investigation notes:

This is closely related to (and arguably a duplicate of) #329.

The root cause is in API/Backend/Config/routes/configs.js. The upsert() function has no early guard against being called with neither version nor config. When an empty config {} is passed without a version:

1. populateUUIDs({}) is called at line 561 — before validate() at line 586
2. populateUUIDs crashes on the missing layers property
3. The generic .catch() at line 692 returns "Failed to find mission." — a misleading error that has nothing to do with the actual problem

The docs (docs/pages/APIs/Configure/Configure_REST_API.md line 120-124) and OpenAPI spec (docs/mmgis-openapi.json line 248-251) both mark version and config as independently optional with no mention that at least one must provide valid data.

Suggested fixes:

• Add an early guard at the top of upsert(): if !hasVersion && (req.body.config == null || Object.keys(req.body.config).length === 0), return a clear error
• Move validate(configJSON) before populateUUIDs(configJSON) so invalid configs get a proper validation error instead of crashing
• Update the catch-all error at line 692 to include the actual error details, not just "Failed to find mission."
• Update docs to state: "At least one of version or config is required. If both are provided, version takes precedence."

From #333, #329

Read API/Backend/Config/routes/configs.js, especially upsert(), populateUUIDs(), validate(), and the catch block near line 692. Then compare the request requirements in docs/pages/APIs/Configure/Configure_REST_API.md and docs/mmgis-openapi.json. Done means invalid mission-level config receives a specific validation error, empty requests are rejected clearly, and the documented version/config requirement matches the behavior.