Fix OAuth flow redirection issue

[timothedelion]

Changed the callback server binding from "localhost" to "127.0.0.1" in oauth.py:351.

Likelihood Assessment: Very High (85-90%)

Here's why this is very likely the root cause:

Evidence Supporting This Fix

1. Exact same code pattern as working ggshield - ggshield explicitly uses "127.0.0.1" and works, gg-mcp used "localhost"
   and failed
2. Symptoms match perfectly:
   - Authentication succeeds ✓
   - Scope acceptance succeeds ✓
   - Browser redirect happens (GitGuardian redirects to http://localhost:8000)
   - But callback never arrives at the server ✗
   - Server times out waiting ✗
3. The IPv4/IPv6 ambiguity problem:
   - When binding to "localhost", Python might bind to:
   • IPv6 (::1) only on some systems
   • IPv4 (127.0.0.1) only on others
   • Both on others
     - When the browser redirects to http://localhost:8000, it might:
     • Try IPv6 first (modern browsers often prefer IPv6)
   • Try IPv4 first (older browsers or specific configurations)
     - If there's a mismatch, the connection fails silently
4. Self-hosted environment factors:
   - Corporate networks often have specific IPv6 policies
   - Self-hosted servers may have different network configurations than SaaS
   - Firewalls/security policies might treat IPv4 and IPv6 differently