Document and normalize MCP tool filter names

[syedkazmi14]

Title:
Fix MCP tool-name prefix handling in Python SDK

Description:
Fixes #869

This PR documents that MCP tools registered via mcp_servers are exposed to the runtime as <server-key>-<tool-name>, and updates Python tool-filter handling so bare MCP tool names automatically gain matching server-prefixed aliases when the corresponding MCP server exposes them, including wildcard server configs (tools=["*"]).

Changes included:

• Added MCP tool-filter normalization in python/copilot/_mode.py and wired it into the Python session create/resume paths
• Normalized MCP tool filters for available_tools, excluded_tools, custom-agent tools, and default_agent.excluded_tools
• Documented the <server-key>-<tool-name> convention and SDK normalization behavior in python/copilot/session.py and python/README.md
• Added regression coverage in python/test_tool_set.py and python/test_client.py

[SteveSandersonMS]

Thanks for digging into this, and for the detailed writeup in #869. The repro is clear and this is a genuinely confusing failure mode (zero tools, no error, hooks never fire).

Before going further with the code change, I want to flag something that changes the right scope here: the <server-key>-<tool-name> convention is actually already documented in this SDK (and every other language SDK), just not in the place you were looking. See ToolSet.add_mcp() in python/copilot/_mode.py:

def add_mcp(self, tool_name: str) -> ToolSet:
    """Add an MCP tool pattern (e.g. "github-list_issues" or "*")."""

The same docstring/example exists in the nodejs, dotnet, java, go, and rust SDKs too (ToolSet.addMcp() in each). So available_tools=["mcp:my-server-list_tables"] (or ToolSet().add_mcp("my-server-list_tables")) already works correctly today. The gap is that this isn't mentioned in the README or in the SessionConfig/MCPRemoteServerConfig docstrings that people read when passing a plain list[str], which is exactly the scenario in your repro.

Given that, I think the right-sized fix here is docs-only, and should probably span all the SDKs rather than just Python, since the naming convention is a backend/CLI-level thing and every language hits the same trap:

• Add a line to each SDK's README (and the available_tools/excluded_tools/custom_agents/default_agent docstrings) explaining the <server-key>-<tool-name> convention and pointing to ToolSet.addMcp()/mcp: prefix as the reliable way to target MCP tools.

I'd rather not add the auto-alias-expansion logic in _mode.py/client.py. A few reasons:

• It only fixes Python, so JS/Go/Rust/Java/.NET users still hit the exact same silent failure.
• It changes runtime behavior silently (e.g., an excluded_tools entry that previously no-op'd against an MCP tool would now actually start excluding it), which could be a surprising behavior change for existing callers.
• It's solving via a guess (matching against configured mcp_servers.tools) something the SDK already lets users express exactly and unambiguously via mcp:<server-key>-<tool-name>.

Would you be up for narrowing this PR to the doc changes (README + docstrings) and applying the same wording across the other SDK directories? This is a good contribution, just want to make sure it lands in the most maintainable spot.