Go SDK: Upgrade MCP SDK and remove vendored copy (#35)

<!-- Provide a brief summary of your changes -->
Update the Go interceptor SDK to use
`github.com/modelcontextprotocol/go-sdk` v1.7.0 custom method APIs and
remove the vendored Go SDK copy. This shouldn't update the behaviours,
but addding following functions.

Changed public/user-facing interfaces:
extension.Extension.Install now returns an error:

```
// before
ext.Install(mcpServer)

// after
if err := ext.Install(mcpServer); err != nil {
	return err
}
```

New helper added:

```
extension.RegisterSendingMethods(client *mcp.Client) error
```

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->

Pick up MCP Go SDK 1.7.0. This provides upstream custom method support,
so the local vendored SDK copy is no longer needed.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

- `go test -v ./...`

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

```go
if err := ext.Install(mcpServer); err != nil {
    return err
}
```

The Go SDK module now requires Go 1.25 via
github.com/modelcontextprotocol/go-sdk v1.7.0.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->
LocalChain now registers interceptor custom sending methods internally.
Manual remote chain setup should call
`extension.RegisterSendingMethods(client)` before connecting the MCP
client session used with chain.AddMCPServer.

Signed-off-by: Ukjae Jeong <jeongukjae@gmail.com>

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

@@ -16,11 +16,13 @@
 //
 // # Usage
 //
-//	// Create interceptor extension (registers interceptors as resources)
+//	// Create interceptor extension (exposes interceptors as custom methods)
 //	ext := extension.New()
 //	ext.AddInterceptor(myValidator)
 //	ext.AddInterceptor(myMutator)
-//	ext.Install(mcpServer)
+//	if err := ext.Install(mcpServer); err != nil {
+//	    return err
+//	}
 //
 //	// Create chain via in-memory transport
 //	chain, err := ext.LocalChain(ctx, mcpServer)

go/sdk/interceptors/integrations/gomiddleware/middleware.go

@@ -66,7 +66,7 @@ var skipMethods = map[string]bool{
 // interceptor/invoke RPCs. Install it on an [mcp.Server] via
 // [mcp.Server.AddReceivingMiddleware]:
 //
-//	chain, _ := srv.LocalChain(ctx)
+//	chain, _ := ext.LocalChain(ctx, mcpServer)
 //	mcpServer.AddReceivingMiddleware(
 //	    gomiddleware.Middleware(chain, gomiddleware.WithLogger(logger)),
 //	)

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

@@ -41,7 +41,7 @@ func setup(t *testing.T, is ...interceptors.Interceptor) *mcp.ClientSession {
 	for _, i := range is {
 		ext.AddInterceptor(i)
 	}
-	ext.Install(mcpServer)
+	require.NoError(t, ext.Install(mcpServer))
 
 	// Create chain via LocalChain (in-memory transport).
 	chain, err := ext.LocalChain(context.Background(), mcpServer)
@@ -267,7 +267,7 @@ func TestMiddlewareWithContextProvider(t *testing.T) {
 
 	ext := extension.New()
 	ext.AddInterceptor(principalCheck)
-	ext.Install(mcpServer)
+	require.NoError(t, ext.Install(mcpServer))
 
 	chain, err := ext.LocalChain(context.Background(), mcpServer)
 	require.NoError(t, err)