Python: Warn when advertised MCP archives are rejected

[Ruiming Zhao (uuzzrm)]

Motivation & Context

MCPSkillsSource can advertise an archive skill in skill://index.json and then return fewer skills when that archive is rejected during download or materialization. The refusal was only logged at DEBUG, so operators could not distinguish a missing skill from one that was never published.

This change makes refusals of well-formed, advertised archive entries visible at WARNING. It keeps malformed index entries and MCP -32002 missing-resource responses at DEBUG because those cases can be expected during capability discovery.

Description & Review Guide

• What are the major changes?
  • Promote archive refusal logs for empty/non-binary content, download-size limits, unsupported formats, missing SKILL.md, invalid UTF-8, and frontmatter name mismatches from DEBUG to WARNING.
  • Add a regression test covering the frontmatter name mismatch path.
• What is the impact of these changes?
  • There is no API or behavior change: rejected archives are still skipped, but the reason is visible at the default operational log level.
  • Archive safety limits and validation remain unchanged.
• What do you want reviewers to focus on?
  • Whether the WARNING/DEBUG boundary is appropriate for advertised archive entries versus malformed or unavailable index resources.

AI assistance was used to help draft the patch and regression test; I reviewed the resulting diff and ran the relevant test suites.

Related Issue

Fixes #7619

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Promotes rejected advertised MCP archive logs to WARNING without changing skill-loading behavior.

Changes:

• Raises archive refusal visibility from DEBUG to WARNING.
• Adds regression coverage for frontmatter name mismatches.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/core/agent_framework/_skills.py	Promotes archive rejection logs.
python/packages/core/tests/core/test_mcp_skills.py	Verifies mismatch warnings.

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