Add unit tests for RTSPHeader.SplitHeaders line folding (#1657)

SplitHeaders performs RTSP header line-folding (collapsing continuation
lines that begin with whitespace) and normalisation of malformed
carriage-return line endings, but had no direct test coverage; it was
only exercised indirectly via well-formed round-trip messages.

These characterisation tests lock in the existing behaviour:
- basic CRLF-separated splitting
- continuation line folded with a leading space
- continuation line folded with a leading tab
- lone "\r " (CR + space) treated as a line break

Verified to pass against both the current regex-based implementation and
the span-based rewrite in #1654, confirming the two are equivalent.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: sipsorcery

test/unit/net/RTSP/RTSPHeaderUnitTest.cs

@@ -0,0 +1,112 @@
+//-----------------------------------------------------------------------------
+// Filename: RTSPHeaderUnitTest.cs
+//
+// Description: Unit tests for the RTSPHeader class, in particular the header
+// splitting/line-folding logic in SplitHeaders.
+//
+// Author(s):
+// Aaron Clauson
+//
+// History:
+// 01 Jun 2026	Aaron Clauson	Created to characterise SplitHeaders line folding.
+//
+// License:
+// BSD 3-Clause "New" or "Revised" License, see included LICENSE.md file.
+//-----------------------------------------------------------------------------
+
+using Microsoft.Extensions.Logging;
+using SIPSorcery.UnitTests;
+using Xunit;
+
+namespace SIPSorcery.Net.UnitTests
+{
+    [Trait("Category", "unit")]
+    public class RTSPHeaderUnitTest
+    {
+        private const string CRLF = "\r\n";
+
+        private Microsoft.Extensions.Logging.ILogger logger = null;
+
+        public RTSPHeaderUnitTest(Xunit.Abstractions.ITestOutputHelper output)
+        {
+            logger = SIPSorcery.UnitTests.TestLogHelper.InitTestLogger(output);
+        }
+
+        /// <summary>
+        /// Tests that a well formed block of CRLF separated headers is split into one
+        /// entry per header line.
+        /// </summary>
+        [Fact]
+        public void SplitHeadersBasicTest()
+        {
+            logger.LogDebug("--> {MethodName}", TestHelper.GetCurrentMethodName());
+            logger.BeginScope(TestHelper.GetCurrentMethodName());
+
+            string message = "CSeq: 1" + CRLF + "Session: abc123" + CRLF + "Content-Length: 0";
+
+            string[] headers = RTSPHeader.SplitHeaders(message);
+
+            Assert.Equal(3, headers.Length);
+            Assert.Equal("CSeq: 1", headers[0]);
+            Assert.Equal("Session: abc123", headers[1]);
+            Assert.Equal("Content-Length: 0", headers[2]);
+        }
+
+        /// <summary>
+        /// Tests that a header value folded across multiple lines (a continuation line begins
+        /// with whitespace) is collapsed back onto a single line separated by a single space.
+        /// </summary>
+        [Fact]
+        public void SplitHeadersFoldedWithSpaceTest()
+        {
+            logger.LogDebug("--> {MethodName}", TestHelper.GetCurrentMethodName());
+            logger.BeginScope(TestHelper.GetCurrentMethodName());
+
+            string message = "Accept: application/sdp," + CRLF + " application/rtsl" + CRLF + "CSeq: 2";
+
+            string[] headers = RTSPHeader.SplitHeaders(message);
+
+            Assert.Equal(2, headers.Length);
+            Assert.Equal("Accept: application/sdp, application/rtsl", headers[0]);
+            Assert.Equal("CSeq: 2", headers[1]);
+        }
+
+        /// <summary>
+        /// Tests that a continuation line indented with a tab is also folded back onto the
+        /// previous line.
+        /// </summary>
+        [Fact]
+        public void SplitHeadersFoldedWithTabTest()
+        {
+            logger.LogDebug("--> {MethodName}", TestHelper.GetCurrentMethodName());
+            logger.BeginScope(TestHelper.GetCurrentMethodName());
+
+            string message = "Accept: a," + CRLF + "\tb" + CRLF + "CSeq: 3";
+
+            string[] headers = RTSPHeader.SplitHeaders(message);
+
+            Assert.Equal(2, headers.Length);
+            Assert.Equal("Accept: a, b", headers[0]);
+            Assert.Equal("CSeq: 3", headers[1]);
+        }
+
+        /// <summary>
+        /// Tests that a malformed line ending of a lone carriage return followed by a space
+        /// (some user agents don't get the \r\n right) is treated as a proper line break.
+        /// </summary>
+        [Fact]
+        public void SplitHeadersMalformedCarriageReturnTest()
+        {
+            logger.LogDebug("--> {MethodName}", TestHelper.GetCurrentMethodName());
+            logger.BeginScope(TestHelper.GetCurrentMethodName());
+
+            string message = "CSeq: 4\r Session: xyz";
+
+            string[] headers = RTSPHeader.SplitHeaders(message);
+
+            Assert.Equal(2, headers.Length);
+            Assert.Equal("CSeq: 4", headers[0]);
+            Assert.Equal("Session: xyz", headers[1]);
+        }
+    }
+}