Add MCP-based skills discovery (McpSkill, McpSkillsSource, McpSkillResource)

Implement Agent Skills discovery over MCP following the SEP-2640 convention:
- McpSkillsSource: reads skill://index.json to discover skills served by an MCP server
- McpSkill: lazily fetches SKILL.md content via resources/read on demand
- McpSkillResource: wraps MCP resource results (text and binary)
- Path traversal protection in get_resource for defense in depth
- Samples for Foundry Toolbox and standalone MCP skills server
- Comprehensive unit tests (514 lines)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/samples/02-agents/skills/mcp_based_skill/README.md

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+# MCP-Based Agent Skills Sample
+
+This sample demonstrates how to discover **Agent Skills served over MCP** with an `Agent`.
+
+## What it demonstrates
+
+- Connecting to a remote MCP server (over streamable HTTP) that exposes skill
+  resources following the SEP-2640 convention.
+- Building a `SkillsProvider` from an `McpSkillsSource`, which reads
+  `skill://index.json` (SEP-2640 canonical discovery) and constructs skills from
+  the index entries.
+- The progressive disclosure pattern across MCP: advertise → load → read
+  resources, exactly as for filesystem-backed skills.
+
+## Running the Sample
+
+### Prerequisites
+
+- Python 3.10+
+- An [Azure AI Foundry](https://ai.azure.com/) project with a deployed model
+- Azure CLI authentication (`az login`)
+- A running MCP server that hosts SEP-2640 skill resources (see "Providing
+  an MCP server" below)
+
+### Setup
+
+Set the following environment variables (in a `.env` file or your shell):
+
+```powershell
+$env:FOUNDRY_PROJECT_ENDPOINT="https://your-endpoint.services.ai.azure.com/api/projects/your-project"
+$env:FOUNDRY_MODEL="gpt-4o-mini"
+$env:MCP_SKILLS_SERVER_URL="https://your-mcp-server.example.com/mcp"
+```
+
+### Run
+
+```powershell
+python mcp_based_skill.py
+```
+
+## Providing an MCP server
+
+This sample is a **consumer**: it does not host an MCP server itself. To try
+it end-to-end you need an MCP server that exposes the SEP-2640 skill
+resources (`skill://index.json` plus per-skill `SKILL.md`).
+
+- See [`samples/02-agents/mcp/agent_as_mcp_server.py`](../../mcp/agent_as_mcp_server.py)
+  for an example of hosting an MCP server via the Agent Framework.
+- The Model Context Protocol working group maintains reference MCP-skills
+  servers at
+  [`modelcontextprotocol/experimental-ext-skills`](https://github.com/modelcontextprotocol/experimental-ext-skills).