feat: Add roots enforcement utility to FastMCP (get_roots, assert_within_roots, @within_roots_check)

[NeelakandanNC]

Description

Problem

FastMCP has no built-in way to enforce that file paths or resource URIs stay
within client-declared roots. The MCP spec defines a Roots capability so clients
can advertise which filesystem boundaries a server should respect — but nothing
in the SDK enforces those boundaries.

Server authors who want to honour roots today must:

• Manually call ctx.session.list_roots() on every tool invocation
• Write their own path normalisation and prefix-checking logic
• Repeat this across every tool that handles file paths

This makes it easy to accidentally bypass the security intent of the Roots
capability.

Example of the gap

# Client declares roots: ["/home/user/project"]
# But a tool call arrives with an arbitrary path:

@mcp.tool()
async def read_file(path: str, ctx: Context) -> str:
    with open(path) as f:           # No enforcement.
        return f.read()              # Happily reads /etc/passwd
                                     # or ../../.ssh/id_rsa.

The client declared a boundary. The SDK exposed that boundary via ctx.session.list_roots(). But nothing checked the incoming path against it — because that check is the server author's responsibility, and the SDK ships no helper for it.

Proposed Solution

Add a utility module at src/mcp/server/fastmcp/utilities/roots.py that exposes:

get_roots(ctx) — async helper that fetches the current roots from the session.

assert_within_roots(path, roots) — validates a path is within at least one
declared root; raises PermissionError with a clear message if not.

@within_roots_check — decorator for tool functions that runs the above two
automatically, so a tool can be protected with one line.

Example

from mcp.server.fastmcp import FastMCP
from mcp.server.fastmcp.utilities.roots import within_roots_check

mcp = FastMCP("file-server")

@mcp.tool()
@within_roots_check
async def read_file(path: str, ctx: Context) -> str:
    with open(path) as f:
        return f.read()

If path is outside any client-declared root, the decorator raises before the
body runs.

Implementation Notes

• Roots are fetched via ctx.session.list_roots() (the existing SDK pattern)
• Path comparison uses pathlib.Path.resolve() to normalise symlinks and
  relative segments before prefix-checking
• The decorator uses functools.wraps to preserve the tool function's signature
• Follows the same pattern as other FastMCP utilities (logging, dependencies)

Checklist

• Searched existing issues — no duplicate found
• Implementation and tests written locally on branch feat/roots-utility
• Will submit a PR referencing this issue

References

No response