fix authenticated_server_python WWW-Authenticate formatting

[chi5477]

Summary

This PR fixes a formatting bug in the OAuth challenge string returned by the Python authenticated MCP example.

What was wrong

In authenticated_server_python/main.py, the generated WWW-Authenticate challenge concatenated error and error_description without a comma separator.

Before:

• error="..."error_description="..."

This produces a malformed challenge value and can cause OAuth clients to parse the auth hint inconsistently.

Root cause

A missing delimiter in _build_www_authenticate_value(...) when composing challenge parameters.

Fix

Add the missing comma+space between error and error_description in the formatted string.

After:

• error="...", error_description="..."

Validation

• Triggered unauthenticated call to protected tool see_past_orders via /mcp.
• Confirmed mcp/www_authenticate now returns a correctly formatted challenge:
  • Bearer error="invalid_request", error_description="No access token was provided", resource_metadata="..."

Scope

• Minimal change: one-line fix in one file.
• No behavior changes outside authenticated challenge formatting.