.NET: Python: .NET: Make skill resource guidance rule-only

[Sam (samrusani)]

Motivation & Context

AgentSkillsProvider currently places fictional resource names in the default
Python and .NET instructions. A model can mistake those examples for resources
that the loaded skill actually provides and make a guaranteed-to-fail
read_skill_resource call.

This change keeps resource access grounded in the loaded skill while preserving
the existing resource discovery and tool APIs.

Fixes #7663

Description & Review Guide

• What are the major changes?
  • Replace the concrete resource-name examples in both implementations with
    matching rule-only guidance: read only explicitly referenced resources,
    preserve the referenced path exactly, and never infer resource paths.
  • Add focused Python and .NET prompt regressions that reject the fictional
    examples.
  • Add positive controls showing that a real resource path remains advertised
    exactly when the loaded skill explicitly provides it.
• What is the impact of these changes?
  • Default skill instructions no longer prime models with nonexistent resource
    names. Public APIs and valid resource listings are unchanged.
• What do you want reviewers to focus on?
  • Confirm that the three resource rules remain equivalent across Python and
    .NET and that the C# assertions match the existing skills-test conventions.

The focused Python regressions fail against the base prompt and pass with this
change. The complete Python test_skills.py file and core syntax checks pass.
The .NET tests were not run locally because the .NET CLI was unavailable; draft
CI is expected to provide that validation.

Related Issue

Fixes #7663

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

Removes fictional skill resource examples from Python and .NET prompts while preserving explicit resource discovery.

Changes:

• Replaces examples with equivalent rule-only guidance.
• Adds prompt regressions and explicit-path positive controls.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
python/packages/core/agent_framework/_skills.py	Updates Python resource guidance.
python/packages/core/tests/core/test_skills.py	Adds Python regression coverage.
dotnet/src/Microsoft.Agents.AI/Skills/AgentSkillsProvider.cs	Updates .NET resource guidance.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentSkillsProviderTests.cs	Adds .NET regression coverage.

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