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

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🐛 Bug-fix PR

📌 Summary

A successful federated tools/call result may carry all of its data in structuredContent while content is empty. The MCP Tools specification makes unstructured content a SHOULD, not a MUST, when structured content is present. ContextForge's streamable-HTTP transport treated that valid shape as an empty result and returned before reaching the structured-content handler.

For tools declaring an outputSchema, the MCP SDK then substituted:

Output validation error: outputSchema defined but no structured output returned

The real structured payload never reached the client.

Closes #5472

🔁 Reproduction Steps

1. Federate an upstream MCP tool that declares an outputSchema.
2. Return { "content": [], "structuredContent": { ... } } from its tools/call result.
3. Invoke the tool through ContextForge's /mcp streamable-HTTP endpoint.
4. Current main returns the validation-error text instead of forwarding the structured payload.

The issue includes a standalone reproduction repository and before/after output.

🐞 Root Cause

After invoke_tool() returned successfully, the early guard was:

if not result or not result.content:
    return []

It ignored result.structured_content. Structured-only results therefore returned [] before the later handler could assemble (unstructured, structured) for SDK validation.

💡 Fix Description

Treat the result as empty only when both representations are absent:

if not result or (not result.content and not getattr(result, "structured_content", None)):

Structured-only results now fall through to the existing handler and return (unstructured, structured). Results with neither representation continue to return [].

This is intentionally separate from #6182/#6274: those track preservation of an empty dict (structuredContent: {}) in the service layer. This fix handles non-empty structured payloads whose unstructured content array is empty.

📏 Reviewability

• This PR has one clear purpose
• The linked issue is not labeled triage
• Unrelated bugs or improvements are tracked in separate issues/PRs
• Tests are included with the code they validate
• If AI-assisted, I understand and can explain the generated changes

🧪 Verification

Pre-fix: the new structured-only test failed with result == [].

Post-fix on this branch:

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 or preserves_is_error_for_egress'
# 5 passed

Additional checks:

uvx ruff@0.16.1 check mcpgateway/transports/streamablehttp_transport.py
# All checks passed

Full project lint/test/coverage were not duplicated locally due to the large dependency set; CI will provide that result.

Check	Command	Status
Changed source lint	Ruff 0.16.1	Pass
Targeted regression	5 tests	Pass
Pre-fix regression	Current-main behavior	Fail as expected
Full lint / test / coverage	make lint / CI	Pending CI

📐 MCP Compliance (if relevant)

• Matches current MCP spec — unstructured content is a SHOULD when structuredContent is present
• No breaking change to MCP clients
• Existing genuinely-empty results still return []

✅ Checklist

• Code formatted
• No secrets/credentials committed