Python: fix(mcp): do not duplicate structuredContent when content present

[LI (ktz03)]

Description

Fixes #7866. MCP CallToolResult often returns both content and structuredContent. _parse_tool_result_from_mcp previously appended a JSON dump of structuredContent even when content already had items, so the model saw duplicated tool output.

Change: only serialize structuredContent into model-visible Content when content produced no items. Host transports still retain complete structuredContent on the existing Host payload marker.

Validation

• Updated test_parse_tool_result_from_mcp_structured_content_with_text
• Existing test_parse_tool_result_from_mcp_structured_content_only still covers the empty-content fallback

[LI (ktz03)]

Fork CI for this PR may need a maintainer to Approve workflows before Python tests run (same as other community PRs). Happy to address review feedback once checks appear.

[Copilot]

🟡 Changes recommended

Existing test expectations still require the old duplicate-output behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Fixes duplicate MCP tool output when both content and structuredContent are present.

Changes:

• Use structuredContent as model-visible content only when regular content is empty.
• Preserve the complete structured payload for host transports.
• Update parser regression coverage.

File summaries

File	Summary
python/packages/core/agent_framework/_mcp.py	Prevents duplicate structured content. Moderate finding (3 votes): remaining test expectations must be updated.
python/packages/core/tests/core/test_mcp.py	Updates structured-content parsing expectations; several existing expectations still require the old duplicate behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

[LI (ktz03)]

Follow-up for Copilot review: aligned the three suites that still expected duplicated content+structuredContent items with the single-item contract. See 6d58210.

[lsmlhi_25 (FOWEPJF255)]

CI is green on this branch. The scoped MCP change (skip dumping structuredContent when content already has items) looks review-ready and matches the duplication report in #7866.

Happy to help with any follow-up nits if maintainers want a second look.

[Ricky-7-Yan]

I don't think this should hard-code content-first behavior yet.

The issue discussion identified two distinct cases: servers that duplicate the same value in content and structuredContent, and servers where the two fields are complementary. This implementation fixes the former by dropping structuredContent whenever any content block exists, but it also silently removes complementary structured data from the model-visible result.

That is the ambiguity maintainers called out in #7866, where the direction discussed was an explicit selection policy (structured first, content first, content only, structured only, or both), likely with a documented default. Could this expose that policy instead of choosing content-first globally? The tests should cover structured-only, content-only, equivalent-both, complementary-both, and retention of the full payload in HostMessageContent.