Fix panic in test mock handlers with safe type assertions

[Copilot]

Test mock HTTP handlers were panicking on nil type assertions when handling requests without method fields (e.g., SSE heartbeats, connection probes).

Changes

TestCallBackendTool_ReturnsNonNilCallToolResult handler:

• Replace require.NoError(err) with error check + HTTP 400 response (line 32)
• Add safe type assertion for method field (line 37-40)

TestCallBackendTool_ErrorStillReturnsCallToolResult handler:

• Add JSON decode error handling with HTTP 400 response (line 157-161)
• Replace unsafe method := req["method"].(string) with safe check (line 163-167)

Before

var req map[string]interface{}
err := json.NewDecoder(r.Body).Decode(&req)
require.NoError(err)  // ❌ Fails test on EOF

method := req["method"].(string)  // ❌ Panics if nil

After

var req map[string]interface{}
err := json.NewDecoder(r.Body).Decode(&req)
if err != nil {
    w.WriteHeader(http.StatusBadRequest)
    return
}

method, ok := req["method"].(string)
if !ok {
    w.WriteHeader(http.StatusBadRequest)
    return
}

Both handlers now gracefully reject invalid requests instead of panicking or failing the test.

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