Backward Compatible - Http3SettingsFrame abstraction (#15835) (#353)

* [x] Provide a cleaner, more maintainable implementation of
`Http3SettingsFrame`.
* [x] Replace the previous untyped `LongObjectHashMap`-based approach
with a typed `Http3Settings` abstraction.
* [x] Improve API clarity and future extensibility for HTTP/3 settings
handling (similar to Http2Settings)

---

* [x] Introduced `Http3Settings` class to encapsulate and manage HTTP/3
settings.
* [x] Deprecated legacy untyped accessors (`get()`, `put()`) in favor of
typed methods such as `qpackMaxTableCapacity()` and
`enableConnectProtocol()`.
* [x] Updated `DefaultHttp3SettingsFrame` to use the new `Http3Settings`
internally.
* [x] Added detailed Javadoc to guide migration to the new typed API.
* [x] Added comprehensive test coverage for `Http3Settings` and its
integration with `Http3SettingsFrame`.

---

* [x] **Backward-compatible** – existing code using `get()`/`put()`
still works (marked deprecated).
* [x] **Forward-compatible** – encourages use of typed accessors.
* [x] **No behavioral change** – internal structure and API clarity
improved only.

---

Port of netty/netty#15835

Author: normanmaurer

src/main/java/io/netty/incubator/codec/http3/DefaultHttp3SettingsFrame.java

@@ -17,31 +17,64 @@
 
 import io.netty.util.collection.LongObjectHashMap;
 import io.netty.util.collection.LongObjectMap;
+import io.netty.util.internal.ObjectUtil;
 import io.netty.util.internal.StringUtil;
 
+import javax.annotation.Nullable;
 import java.util.Iterator;
 import java.util.Map;
 
 public final class DefaultHttp3SettingsFrame implements Http3SettingsFrame {
 
-    private final LongObjectMap<Long> settings = new LongObjectHashMap<>(4);
+    private final Http3Settings settings;
 
+    public DefaultHttp3SettingsFrame(Http3Settings settings) {
+        this.settings = ObjectUtil.checkNotNull(settings, "settings");
+    }
+
+    public DefaultHttp3SettingsFrame() {
+        this.settings = new Http3Settings();
+    }
+
+    @Override
+    public Http3Settings settings() {
+        return settings;
+    }
+
+    /**
+     * Get a setting by its key.
+     *
+     * @param key the HTTP/3 setting key
+     * @return the value, or {@code null} if not set
+     * @deprecated  use {@link #settings()}  and manipulate the {@link Http3Settings} directly
+     */
     @Override
+    @Deprecated
+    @Nullable
     public Long get(long key) {
+        // Legacy behavior: direct map access.
         return settings.get(key);
     }
 
+    /**
+     * Set a setting value by key.
+     *
+     * @param key the HTTP/3 setting key
+     * @param value the value to set
+     * @return the previous value, or {@code null} if none
+     * @throws IllegalArgumentException if the key is reserved for HTTP/2
+     * @deprecated  use {@link #settings()}  and manipulate the {@link Http3Settings} directly
+     */
     @Override
+    @Deprecated
+    @Nullable
     public Long put(long key, Long value) {
-        if (Http3CodecUtils.isReservedHttp2Setting(key)) {
-            throw new IllegalArgumentException("Setting is reserved for HTTP/2: " + key);
-        }
         return settings.put(key, value);
     }
 
     @Override
     public Iterator<Map.Entry<Long, Long>> iterator() {
-        return settings.entrySet().iterator();
+        return settings.iterator();
     }
 
     @Override