getFirstValue silently returned null when given a non-existent fieldApiName, making it impossible to distinguish a missing field from a null field value.

Changes

• Validation logic (CollectionGetFirstValue_Records.cls): Before calling firstRecord.get(), checks the field exists in the SObject's describe field map. Throws IllegalArgumentException with the invalid field name and SObject type if not found.
• Tests (CollectionGetFirstValue_RecordsTest.cls): Added testGetFirstValue_InvalidFieldApiName verifying the exception type, message content (field name + SObject type), and that no other exception types are thrown.
• README: Updated Behavior Notes to reflect the new exception behavior.

// Before: silently returned null for bad field names
// After: throws with a clear message
// IllegalArgumentException: Invalid field API name 'BadField__xyz' for SObject type 'Account'.
SObject firstRecord = req.records[0];
Schema.SObjectType sObjectType = firstRecord.getSObjectType();
Map<String, Schema.SObjectField> fieldMap = sObjectType.getDescribe().fields.getMap();

if (!fieldMap.containsKey(req.fieldApiName.toLowerCase())) {
    throw new IllegalArgumentException(
        'Invalid field API name \'' + req.fieldApiName + '\' for SObject type \'' + sObjectType.getDescribe().getName() + '\'.'
    );
}