Fix golangci-lint CI failures in internal/proxy/handler.go (#2189)

Two staticcheck violations introduced in `internal/proxy/handler.go`
were causing the lint CI job to fail.

## Changes

- **Remove `forwardGraphQL`**: Dead code — defined but never called.
GraphQL forwarding is handled inline in `handleWithDIFC`.
- **Fix S1034 type switch**: Eliminate redundant type assertion inside
the `map[string]interface{}` case by using the idiomatic assigned type
switch form:

```go
// Before
switch originalData.(type) {
case map[string]interface{}:
    obj := originalData.(map[string]interface{}) // redundant assertion
    ...
}

// After
switch obj := originalData.(type) {
case map[string]interface{}:
    // obj is already the asserted type
    ...
}
```

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `example.com`
> - Triggering command: `/tmp/go-build1696049945/b333/launcher.test
/tmp/go-build1696049945/b333/launcher.test
-test.testlogfile=/tmp/go-build1696049945/b333/testlog.txt
-test.paniconexit0 -test.timeout=10m0s
/tmp/go-build1696049945/b238/vet.cfg go o x_amd64/compile` (dns block)
> - `invalid-host-that-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build1696049945/b318/config.test
/tmp/go-build1696049945/b318/config.test
-test.testlogfile=/tmp/go-build1696049945/b318/testlog.txt
-test.paniconexit0 -test.timeout=10m0s ncod�� 64/src/runtime/cgo
64/src/encoding/asn1/asn1.go x_amd64/asm` (dns block)
> - `nonexistent.local`
> - Triggering command: `/tmp/go-build1696049945/b333/launcher.test
/tmp/go-build1696049945/b333/launcher.test
-test.testlogfile=/tmp/go-build1696049945/b333/testlog.txt
-test.paniconexit0 -test.timeout=10m0s
/tmp/go-build1696049945/b238/vet.cfg go o x_amd64/compile` (dns block)
> - `slow.example.com`
> - Triggering command: `/tmp/go-build1696049945/b333/launcher.test
/tmp/go-build1696049945/b333/launcher.test
-test.testlogfile=/tmp/go-build1696049945/b333/testlog.txt
-test.paniconexit0 -test.timeout=10m0s
/tmp/go-build1696049945/b238/vet.cfg go o x_amd64/compile` (dns block)
> - `this-host-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build1696049945/b342/mcp.test
/tmp/go-build1696049945/b342/mcp.test
-test.testlogfile=/tmp/go-build1696049945/b342/testlog.txt
-test.paniconexit0 -test.timeout=10m0s
/tmp/go-build1696049945/b215/vet.cfg p/go-build linux.go x_amd64/compile
--exclude-hidden/opt/hostedtoolcache/go/1.25.8/x64/pkg/tool/linux_amd64/vet
terpreter --quiet x_amd64/compile 2599�� ache/go/1.25.8/x-errorsas
AcPk/Gl1SDLLxjjl-ifaceassert x_amd64/vet` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/github/gh-aw-mcpg/settings/copilot/coding_agent)
(admins only)
>
> </details>

<!-- START COPILOT CODING AGENT SUFFIX -->



<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> Fix the failing GitHub Actions workflow lint
> Analyze the workflow logs, identify the root cause of the failure, and
implement a fix.
> Job ID: 67821217215
> Job URL:
https://github.com/github/gh-aw-mcpg/actions/runs/23317651473/job/67821217215


</details>



<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

Author: lpcox

internal/proxy/handler.go

@@ -291,24 +291,6 @@ func (h *proxyHandler) passthrough(w http.ResponseWriter, r *http.Request, path
 	h.writeResponse(w, resp, respBody)
 }
 
-// forwardGraphQL forwards a GraphQL request without DIFC filtering.
-func (h *proxyHandler) forwardGraphQL(w http.ResponseWriter, r *http.Request, _ string, body []byte) {
-	resp, err := h.server.forwardToGitHub(r.Context(), http.MethodPost, "/graphql", bytes.NewReader(body), "application/json")
-	if err != nil {
-		http.Error(w, "upstream request failed", http.StatusBadGateway)
-		return
-	}
-	defer resp.Body.Close()
-
-	respBody, err := io.ReadAll(resp.Body)
-	if err != nil {
-		http.Error(w, "failed to read upstream response", http.StatusBadGateway)
-		return
-	}
-
-	h.writeResponse(w, resp, respBody)
-}
-
 // writeResponse writes an upstream response to the client.
 func (h *proxyHandler) writeResponse(w http.ResponseWriter, resp *http.Response, body []byte) {
 	copyResponseHeaders(w, resp)
@@ -326,11 +308,10 @@ func (h *proxyHandler) writeEmptyResponse(w http.ResponseWriter, resp *http.Resp
 	w.WriteHeader(resp.StatusCode)
 
 	var empty string
-	switch originalData.(type) {
+	switch obj := originalData.(type) {
 	case []interface{}:
 		empty = "[]"
 	case map[string]interface{}:
-		obj := originalData.(map[string]interface{})
 		// GraphQL responses wrap their payload in a "data" key
 		if _, ok := obj["data"]; ok {
 			empty = `{"data":null}`