Bulk-safe request processing with per-request error isolation

[Copilot]

A single invalid fieldApiName in a batch would throw an unhandled SObjectException, failing the entire invocation — violating the core bulkification contract.

Changes

• fieldApiName guard — String.isBlank check short-circuits before field access; returns null response with descriptive errorMessage
• try/catch (SObjectException) per request — invalid field names populate errorMessage for that request only; remaining requests in the batch are unaffected
• errorMessage output variable — new @InvocableVariable on Response surfaces per-request errors back to Flow

// Before: this kills every request in the batch
reqBad.fieldApiName = 'NonExistentField__xyz'; // throws SObjectException → entire list fails

// After: bad request gets null response + errorMessage; good requests proceed normally
responses[1].response      // null
responses[1].errorMessage  // "Invalid field NonExistentField__xyz for ..."
responses[2].response      // "Good Account"  ← unaffected

• New tests — testGetFirstValue_BlankFieldApiName, testGetFirstValue_InvalidFieldApiName_DoesNotBreakBatch