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>

.github/workflows/go.yml

@@ -51,7 +51,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.24", "1.25"]
+        go: ["1.25", "1.26"]
     steps:
       - name: Check out code
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
@@ -73,8 +73,8 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
-          go-version: "1.24"
+          go-version: "1.25"
       - name: Test with -race
         run: |
           cd go/sdk
-          go test -v -race ./...
+          go test -v -race ./...