feat: add NatsHeaderParser for MIME header parsing

tests/NATS.Server.Tests/NatsHeaderParserTests.cs

@@ -0,0 +1,51 @@
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Tests;
+
+public class NatsHeaderParserTests
+{
+    [Fact]
+    public void Parse_status_line_only()
+    {
+        var input = "NATS/1.0 503\r\n\r\n"u8;
+        var result = NatsHeaderParser.Parse(input);
+        result.Status.ShouldBe(503);
+        result.Description.ShouldBeEmpty();
+        result.Headers.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void Parse_status_with_description()
+    {
+        var input = "NATS/1.0 503 No Responders\r\n\r\n"u8;
+        var result = NatsHeaderParser.Parse(input);
+        result.Status.ShouldBe(503);
+        result.Description.ShouldBe("No Responders");
+    }
+
+    [Fact]
+    public void Parse_headers_with_values()
+    {
+        var input = "NATS/1.0\r\nFoo: bar\r\nBaz: qux\r\n\r\n"u8;
+        var result = NatsHeaderParser.Parse(input);
+        result.Status.ShouldBe(0);
+        result.Headers["Foo"].ShouldBe(["bar"]);
+        result.Headers["Baz"].ShouldBe(["qux"]);
+    }
+
+    [Fact]
+    public void Parse_multi_value_header()
+    {
+        var input = "NATS/1.0\r\nX-Tag: a\r\nX-Tag: b\r\n\r\n"u8;
+        var result = NatsHeaderParser.Parse(input);
+        result.Headers["X-Tag"].ShouldBe(["a", "b"]);
+    }
+
+    [Fact]
+    public void Parse_invalid_returns_defaults()
+    {
+        var input = "GARBAGE\r\n\r\n"u8;
+        var result = NatsHeaderParser.Parse(input);
+        result.Status.ShouldBe(-1);
+    }
+}