Fix SPARQLResultsTSVWriter to quote xsd:string literals

Related to #5256

Update `SPARQLResultsTSVWriter` to ensure all `xsd:string` literals are quoted in SPARQL TSV results.

* Modify `writeLiteral` method to always quote `xsd:string` literals.
* Remove conditions that allowed `xsd:string` literals to be written without quotes.
* Ensure special characters in `xsd:string` literals are properly escaped using `encodeString` method.
* Update tests in `SPARQLTSVCustomTest` to verify correct quoting of `xsd:string` literals and proper escaping of special characters.

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Author: hmottestad

core/queryresultio/text/src/main/java/org/eclipse/rdf4j/query/resultio/text/tsv/SPARQLResultsTSVWriter.java

@@ -194,21 +194,15 @@ private void writeLiteral(Literal lit) throws IOException {
 			// Append the literal's language
 			writer.write("@");
 			writer.write(lit.getLanguage().get());
-		} else if (!XSD.STRING.equals(datatype) || !xsdStringToPlainLiteral()) {
-			writer.write("\"");
-			writer.write(encoded);
-			writer.write("\"");
-			// Append the literal's datatype
-			writer.write("^^");
-			writeURI(datatype);
-		} else if (!label.isEmpty() && encoded.equals(label) && label.charAt(0) != '<' && label.charAt(0) != '_'
-				&& !label.matches("^[\\+\\-]?[\\d\\.].*")) {
-			// no need to include double quotes
-			writer.write(encoded);
 		} else {
 			writer.write("\"");
 			writer.write(encoded);
 			writer.write("\"");
+			// Append the literal's datatype if it's not xsd:string or if xsdStringToPlainLiteral is false
+			if (!XSD.STRING.equals(datatype) || !xsdStringToPlainLiteral()) {
+				writer.write("^^");
+				writeURI(datatype);
+			}
 		}
 	}
 

core/queryresultio/text/src/test/java/org/eclipse/rdf4j/query/resultio/text/tsv/SPARQLTSVCustomTest.java

@@ -69,6 +69,24 @@ public void testSES2126QuotedLiteralIntegerAsStringImplicitType() throws Excepti
 		assertEquals("?test\n\"1\"\n", result);
 	}
 
+	@Test
+	public void testQuotedXSDStringLiteral() throws Exception {
+		List<String> bindingNames = List.of("test");
+		TupleQueryResult tqr = new IteratingTupleQueryResult(bindingNames,
+				List.of(new ListBindingSet(bindingNames, SimpleValueFactory.getInstance().createLiteral("example", XSD.STRING))));
+		String result = writeTupleResult(tqr);
+		assertEquals("?test\n\"example\"\n", result);
+	}
+
+	@Test
+	public void testQuotedXSDStringLiteralWithSpecialCharacters() throws Exception {
+		List<String> bindingNames = List.of("test");
+		TupleQueryResult tqr = new IteratingTupleQueryResult(bindingNames,
+				List.of(new ListBindingSet(bindingNames, SimpleValueFactory.getInstance().createLiteral("example\twith\nspecial\"characters", XSD.STRING))));
+		String result = writeTupleResult(tqr);
+		assertEquals("?test\n\"example\\twith\\nspecial\\\"characters\"\n", result);
+	}
+
 	private String writeTupleResult(TupleQueryResult tqr)
 			throws IOException, TupleQueryResultHandlerException, QueryEvaluationException {
 		ByteArrayOutputStream output = new ByteArrayOutputStream();