fix(appsec): strip surrounding quotes from charset parameter in MultipartContentDecoder

RFC 2045 allows quoted parameter values (charset="UTF-8"). Without stripping
the quotes Charset.forName rejects the name and decodeBytes falls back to the
JVM default instead of the declared charset.

Author: jandro996

internal-api/src/main/java/datadog/trace/api/http/MultipartContentDecoder.java

@@ -29,6 +29,9 @@ public static Charset extractCharset(String contentType) {
     int idx = contentType.toLowerCase(Locale.ROOT).indexOf("charset=");
     if (idx < 0) return null;
     String name = contentType.substring(idx + 8).split("[;, ]")[0].trim();
+    if (name.length() > 1 && name.charAt(0) == '"' && name.charAt(name.length() - 1) == '"') {
+      name = name.substring(1, name.length() - 1);
+    }
     try {
       return Charset.forName(name);
     } catch (IllegalArgumentException e) {

internal-api/src/test/java/datadog/trace/api/http/MultipartContentDecoderTest.java

@@ -113,4 +113,23 @@ void extractCharsetHandlesAdditionalParameters() {
         MultipartContentDecoder.extractCharset("text/plain; charset=UTF-8; boundary=something")
             .name());
   }
+
+  @ParameterizedTest
+  @CsvSource({
+    "text/plain; charset=\"UTF-8\", UTF-8",
+    "text/xml; charset=\"ISO-8859-1\", ISO-8859-1"
+  })
+  void extractCharsetHandlesQuotedCharsetValue(String contentType, String expectedCharset) {
+    assertEquals(expectedCharset, MultipartContentDecoder.extractCharset(contentType).name());
+  }
+
+  @Test
+  void decodeBytesUsesQuotedDeclaredCharset() {
+    String text = "café";
+    byte[] bytes = text.getBytes(StandardCharsets.ISO_8859_1);
+    assertEquals(
+        text,
+        MultipartContentDecoder.decodeBytes(
+            bytes, bytes.length, "text/plain; charset=\"ISO-8859-1\""));
+  }
 }