fix(invoice): fetch invoice lines from separate API endpoint

Problem:
get_invoice_by_id() returned invoices WITHOUT lines, making it
impossible to analyze invoice details (e.g., filtering by service
types for statistics).

Root Cause:
Dolibarr REST API separates invoice data into two endpoints:
- GET /invoices/{id}       → header only (no lines)
- GET /invoices/{id}/lines → invoice line items

The MCP client only called the header endpoint.

Solution:
- Fetch lines separately via /invoices/{id}/lines endpoint
- Merge lines into invoice response
- Add error handling for backward compatibility with older
  Dolibarr versions that might not support the lines endpoint

Fixes issue reported in GitHub PR #3 comment
https://github.com/latinogino/dolibarr-mcp/pull/3#issuecomment-3906147396

Impact:
Enables detailed invoice analysis, e.g.:
- Filter invoices by line item descriptions
- Calculate statistics per service type
- Analyze quantities and prices per position

src/dolibarr_mcp/dolibarr_client.py

@@ -627,8 +627,28 @@ class DolibarrClient:
         return result if isinstance(result, list) else []
     
     async def get_invoice_by_id(self, invoice_id: int) -> Dict[str, Any]:
-        """Get specific invoice by ID."""
-        return await self.request("GET", f"invoices/{invoice_id}")
+        """Get specific invoice by ID including invoice lines.
+        
+        Note: Dolibarr API separates invoice header and lines into different endpoints.
+        This method combines both to provide complete invoice data including lines.
+        """
+        invoice = await self.request("GET", f"invoices/{invoice_id}")
+        
+        # Fetch invoice lines separately (Dolibarr API design)
+        try:
+            lines = await self.request("GET", f"invoices/{invoice_id}/lines")
+            invoice['lines'] = lines if isinstance(lines, list) else []
+        except DolibarrAPIError as e:
+            # If lines endpoint fails, return invoice without lines
+            # This ensures backward compatibility with older Dolibarr versions
+            self.logger.warning(
+                "Failed to fetch lines for invoice %s: %s",
+                invoice_id,
+                str(e)
+            )
+            invoice['lines'] = []
+        
+        return invoice
     
     async def create_invoice(
         self,