Make McpHttpHeaders internal via shared source

Move McpHttpHeaders.cs from src/ModelContextProtocol.Core/Protocol/ to
src/Common/ and change from public to internal. Add Compile Include links
in ModelContextProtocol.Core, ModelContextProtocol.AspNetCore, and both
test projects so each assembly gets its own internal copy. This follows
the existing shared source pattern used by Experimentals.cs, Throw.cs, etc.

Author: Tarek Mahmoud Sayed

…Protocol.Core/Protocol/McpHttpHeaders.cs src/Common/McpHttpHeaders.cssrc/ModelContextProtocol.Core/Protocol/McpHttpHeaders.cs renamed to src/Common/McpHttpHeaders.cs src/ModelContextProtocol.Core/Protocol/McpHttpHeaders.cs renamed to src/Common/McpHttpHeaders.cs

@@ -7,7 +7,7 @@ namespace ModelContextProtocol.Protocol;
 /// Per RFC 9110, HTTP header names are case-insensitive. Clients and servers must
 /// use case-insensitive comparisons when processing these headers.
 /// </remarks>
-public static class McpHttpHeaders
+internal static class McpHttpHeaders
 {
     /// <summary>
     /// The minimum protocol version that requires standard MCP request headers.
@@ -57,7 +57,7 @@ public static class McpHttpHeaders
     public const string ParamPrefix = "Mcp-Param-";
 
     /// <summary>
-    /// Key used in <see cref="JsonRpcMessageContext.Items"/> to store the <see cref="Tool"/>
+    /// Key used in <see cref="JsonRpcMessageContext.Items"/> to store the tool
     /// definition for the current request, enabling the transport to add custom parameter headers.
     /// </summary>
     internal const string ToolContextKey = "Mcp.Tool";

src/ModelContextProtocol.AspNetCore/ModelContextProtocol.AspNetCore.csproj

@@ -23,6 +23,7 @@
   <ItemGroup>
     <Compile Include="..\Common\Experimentals.cs" Link="Experimentals.cs" />
     <Compile Include="..\Common\Obsoletions.cs" Link="Obsoletions.cs" />
+    <Compile Include="..\Common\McpHttpHeaders.cs" Link="McpHttpHeaders.cs" />
   </ItemGroup>
 
   <ItemGroup>

src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj

@@ -26,6 +26,7 @@
     <Compile Include="..\Common\Obsoletions.cs" Link="Obsoletions.cs" />
     <Compile Include="..\Common\Experimentals.cs" Link="Experimentals.cs" />
     <Compile Include="..\Common\EncodingUtilities.cs" Link="EncodingUtilities.cs" />
+    <Compile Include="..\Common\McpHttpHeaders.cs" Link="McpHttpHeaders.cs" />
     <Compile Include="..\Common\HttpResponseMessageExtensions.cs" Link="HttpResponseMessageExtensions.cs" />
     <Compile Include="..\Common\ServerSentEvents\**\*.cs" Link="ServerSentEvents\%(RecursiveDir)%(FileName)%(Extension)" />
   </ItemGroup>

tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj

@@ -23,6 +23,7 @@
 
   <ItemGroup>
     <Compile Include="..\Common\**\*.cs" />
+    <Compile Include="..\..\src\Common\McpHttpHeaders.cs" Link="McpHttpHeaders.cs" />
   </ItemGroup>
 
   <ItemGroup>

tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj

@@ -39,6 +39,7 @@
     <Compile Include="..\..\src\ModelContextProtocol.Core\Server\InMemoryMcpTaskStore.cs" Link="Server\InMemoryMcpTaskStore.cs" />
     <!-- Include dependencies for the linked InMemoryMcpTaskStore.cs -->
     <Compile Include="..\..\src\Common\Experimentals.cs" Link="Experimentals.cs" />
+    <Compile Include="..\..\src\Common\McpHttpHeaders.cs" Link="McpHttpHeaders.cs" />
   </ItemGroup>
 
   <ItemGroup>