Rename mcpserver package to extension, remove *mcp.Server field (#11)

## Summary

- Rename `interceptors/mcpserver/` → `interceptors/extension/`,
replacing the `Server` wrapper with a lightweight `Extension` that can
install itself on any `*mcp.Server`
- Remove the stored `*mcp.Server` field — `Extension` is now a pure
interceptor container with no server reference
- Add `Install(server)` method to register JSON-RPC methods and init
middleware on a given server
- Update `LocalChain(ctx)` → `LocalChain(ctx, server)` to accept the
server as a parameter
- Remove `MCPServer()`, `Run()`, and `NewStreamableHTTPHandler()`
wrapper methods — users interact with `mcp.Server` directly
- Update all examples, tests, and documentation to use the new API

## Why

`mcpserver.Server` pretended to *be* the server — it had `Run()`,
`NewStreamableHTTPHandler()`, etc. — but it could never cover the full
`*mcp.Server` API, so users constantly needed `MCPServer()` to unwrap
it.

`Extension` doesn't pretend to be the server. It's a pure interceptor
container that can install itself on any server (or multiple servers)
on the fly. No stored server reference, no wrapping, no delegation
surface to maintain.

### Before

```go
mcpServer := mcp.NewServer(...)
srv := mcpserver.NewServer(mcpServer)
srv.AddInterceptor(v)
chain, err := srv.LocalChain(ctx)
```

### After

```go
mcpServer := mcp.NewServer(...)
ext := extension.New()
ext.AddInterceptor(v)
ext.Install(mcpServer)
chain, err := ext.LocalChain(ctx, mcpServer)
```

Signed-off-by: Kurt Degiorgio <kdegiorgio@bloomberg.net>

go/sdk/interceptors/integrations/gomiddleware/doc.go

@@ -16,13 +16,14 @@
 //
 // # Usage
 //
-//	// Create interceptor server (registers interceptors as resources)
-//	srv := mcpserver.NewServer(mcpServer)
-//	srv.AddInterceptor(myValidator)
-//	srv.AddInterceptor(myMutator)
+//	// Create interceptor extension (registers interceptors as resources)
+//	ext := extension.New()
+//	ext.AddInterceptor(myValidator)
+//	ext.AddInterceptor(myMutator)
+//	ext.Install(mcpServer)
 //
 //	// Create chain via in-memory transport
-//	chain, err := srv.LocalChain(ctx)
+//	chain, err := ext.LocalChain(ctx, mcpServer)
 //
 //	// Install middleware for automatic execution
 //	mcpServer.AddReceivingMiddleware(

go/sdk/interceptors/integrations/gomiddleware/middleware_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
@@ -13,8 +14,8 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors"
+	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/extension"
 	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/integrations/gomiddleware"
-	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/mcpserver"
 )
 
 // setup creates a test server with middleware installed and returns a connected client session.
@@ -36,19 +37,23 @@ func setup(t *testing.T, is ...interceptors.Interceptor) *mcp.ClientSession {
 		}, nil
 	})
 
-	srv := mcpserver.NewServer(mcpServer)
+	ext := extension.New()
 	for _, i := range is {
-		srv.AddInterceptor(i)
+		ext.AddInterceptor(i)
 	}
+	ext.Install(mcpServer)
 
 	// Create chain via LocalChain (in-memory transport).
-	chain, err := srv.LocalChain(context.Background())
+	chain, err := ext.LocalChain(context.Background(), mcpServer)
 	require.NoError(t, err)
 
 	// Install the middleware.
 	mcpServer.AddReceivingMiddleware(gomiddleware.Middleware(chain))
 
-	handler := mcpserver.NewStreamableHTTPHandler(srv, nil)
+	handler := mcp.NewStreamableHTTPHandler(
+		func(r *http.Request) *mcp.Server { return mcpServer },
+		nil,
+	)
 	httpServer := httptest.NewServer(handler)
 	t.Cleanup(httpServer.Close)
 
@@ -260,10 +265,11 @@ func TestMiddlewareWithContextProvider(t *testing.T) {
 		}, nil
 	})
 
-	srv := mcpserver.NewServer(mcpServer)
-	srv.AddInterceptor(principalCheck)
+	ext := extension.New()
+	ext.AddInterceptor(principalCheck)
+	ext.Install(mcpServer)
 
-	chain, err := srv.LocalChain(context.Background())
+	chain, err := ext.LocalChain(context.Background(), mcpServer)
 	require.NoError(t, err)
 
 	mcpServer.AddReceivingMiddleware(gomiddleware.Middleware(chain,
@@ -277,7 +283,10 @@ func TestMiddlewareWithContextProvider(t *testing.T) {
 		}),
 	))
 
-	handler := mcpserver.NewStreamableHTTPHandler(srv, nil)
+	handler := mcp.NewStreamableHTTPHandler(
+		func(r *http.Request) *mcp.Server { return mcpServer },
+		nil,
+	)
 	httpServer := httptest.NewServer(handler)
 	t.Cleanup(httpServer.Close)