Set default maxFieldSectionSize to 16MB (#16088)

Long.MAX_VALUE cannot be encoded as a valid QUIC varint (only 62 value
bits are usable, so Max can be 2^62-1 but JAVA Long MAX = 2^63-1), which
can cause HTTP/3/WebTransport negotiation failures. A large but bounded
default is safer and more compatible across implementations.
as per
[RFC9000](https://datatracker.ietf.org/doc/html/rfc9000?#name-variable-length-integer-enc)
<img width="400" height="300" alt="Screenshot 2025-12-26 at 12 29 19 AM"
src="https://github.com/user-attachments/assets/535048fd-1893-4ffd-bc9e-46c9d802e189"
/>

Changed default maxFieldSectionSize from Long.MAX_VALUE to 16 * 1024 *
1024 (16MB).
Result
Fixes #16087
Uses a QUIC-compliant value and prevents failures during SETTINGS
exchange with peers.

> Note : This did not affect any flow uses `new Http3Settings() `only if
implementations use `Http3Settings.defaultSettings()`

Result:

Port of netty/netty#16088

Author: normanmaurer

src/main/java/io/netty/incubator/codec/http3/Http3Settings.java

@@ -265,7 +265,7 @@ public static Http3Settings defaultSettings() {
         return new Http3Settings()
                 .qpackMaxTableCapacity(0)
                 .qpackBlockedStreams(0)
-                .maxFieldSectionSize(Long.MAX_VALUE)
+                .maxFieldSectionSize(16 * 1024 * 1024)
                 .enableConnectProtocol(false);
     }
 

src/test/java/io/netty/incubator/codec/http3/Http3SettingsTest.java

@@ -46,7 +46,7 @@ void testDefaultSettingsValues() {
         assertEquals(0L, settings.qpackMaxTableCapacity());
         assertEquals(0L, settings.qpackBlockedStreams());
         assertEquals(Boolean.FALSE, settings.connectProtocolEnabled());
-        assertEquals(Long.MAX_VALUE, settings.maxFieldSectionSize());
+        assertEquals(16 * 1024 * 1024, settings.maxFieldSectionSize());
     }
 
     @Test