Add SSRF protection for HTTP client redirects

[maxisbey]

Summary

The SDK creates httpx.AsyncClient with follow_redirects=True but no redirect validation. A malicious MCP server can redirect requests to internal network addresses (http://localhost, http://169.254.169.254, http://metadata.google.internal), enabling SSRF attacks.

Location

src/mcp/shared/_httpx_utils.py — the shared httpx client factory.

Proposed Fix

1. Default: Block HTTPS-to-HTTP downgrades on redirects
2. Provide configurable RedirectPolicy presets:
   • BLOCK_SCHEME_DOWNGRADE (default) — block HTTPS→HTTP redirects
   • ENFORCE_HTTPS (strict) — only allow HTTPS destinations
   • ALLOW_ALL (legacy) — current behavior, no restrictions
3. Consider also blocking redirects to private/link-local IP ranges