Validate SObject types in getFirstRecord input list

[Copilot]

Passing a mixed-type SObject list to getFirstRecord could cause silent casting failures downstream. This adds type homogeneity validation before returning the first record.

Changes

CollectionGetFirstRecord_Records.cls

• Iterates the input list and compares each record's getSObjectType() against the first record's type
• Null records within the list are treated as invalid (trigger the mixed-type path)
• On type mismatch: sets record = null and populates a new errorMessage field
• Added errorMessage as an @InvocableVariable on Response so Flow builders can surface the error

CollectionGetFirstRecord_RecordsTest.cls

• Added testGetFirstRecord_MixedSObjectTypesReturnsNullWithError — passes an Account + Contact in the same list, asserts record is null and errorMessage is set

Behavior

// Mixed types → null record + error message
Request req = new Request();
req.records = new List<SObject>{ account, contact };

Response resp = getFirstRecord(new List<Request>{ req })[0];
// resp.record       → null
// resp.errorMessage → 'Mixed SObject types detected in the input list...'