fix(mcp): preserve structured-only federated tool results

[Ricky-7-Yan]

🐛 Bug-fix PR

📌 Summary

A successful federated tools/call result may carry all data in structuredContent while content is empty. The streamable-HTTP transport treated that valid shape as empty and returned before its existing structured-content handler.

The guard now returns early only when both representations are absent. Structured-only results reach the existing tuple response path, while genuinely empty results keep returning [].

Closes #5472

🔁 Reproduction

1. Federate an upstream tool that declares an outputSchema.
2. Have it return { "content": [], "structuredContent": { ... } }.
3. Invoke it through the /mcp streamable-HTTP endpoint.
4. Before this change the structured payload is discarded and the SDK reports an output-validation error.

📏 Reviewability

• This PR has one clear purpose
• The linked issue is not labeled triage
• Unrelated bugs or improvements are tracked separately
• Tests are included with the code they validate

🏷️ Type of Change

• Bug fix

🧪 Verification

The new regression test was run before the source change and failed with result == [].

After the fix:

python -m pytest -q tests/unit/mcpgateway/transports/test_streamablehttp_transport.py -k "call_tool_success or with_structured_content or structured_only_result or neither_representation_exists or preserves_is_error_for_egress"
5 passed

Ruff 0.15.20 passes for the changed source and the changed test while excluding two unrelated violations already present elsewhere in the 17k-line test module (E303 at line 9639 and PLC2801 at line 17055). The source file passes formatting; the test module has two pre-existing formatting differences outside this change.

✅ Checklist

• Tests added for structured-only and genuinely empty results
• No secrets or credentials committed
• Commit includes the required DCO sign-off

[felixandersen]

FWIW I have been running this patch successfully in my environment. Fixed the issue for me.