fix(auth): handle Azure AD v1 tokens in OAuth claim validation

[ecthelion77]

🐛 Bug-fix PR

🔗 Issue

Closes #4158

📌 Summary

The token claim validation introduced in #3941 produces false-positive blocking errors for Azure AD tokens, preventing valid OAuth gateways from fetching tools. Three issues are fixed:

1. Audience: Azure AD v1 tokens use client_id (GUID) as audience when scopes use {client_id}/.default. The validator now accepts client_id and api://{client_id} as valid audiences alongside the configured resource.

2. Issuer: Azure AD v1 tokens use iss=https://sts.windows.net/{tenant}/ while v2 uses https://login.microsoftonline.com/{tenant}/v2.0. Same tenant, different URL. The validator now detects Entra v1/v2 duality and accepts both when the tenant ID matches.

3. Scopes: OIDC meta-scopes (openid, offline_access, profile, email) and Azure AD /.default scopes are request-time directives that never appear in the access token scp claim. The validator now skips them from the missing-scope check.

🔁 Reproduction Steps

1. Configure a gateway with Azure AD OAuth (authorization_code grant)
2. Set scopes to ["{client_id}/.default", "openid", "offline_access"]
3. Complete OAuth flow → "Fetch Tools" → blocked with audience/issuer/scope errors

🐞 Root Cause

_validate_audience() did not consider client_id as a valid audience.
_validate_issuer() did not handle Azure AD v1/v2 issuer URL duality.
_validate_scopes() treated OIDC meta-scopes and /.default as required token claims.

💡 Fix Description

• _validate_audience(): build a set of acceptable audiences including client_id and api://{client_id}
• _validate_issuer(): extract tenant from v1 (sts.windows.net/{tenant}) and v2 (login.microsoftonline.com/{tenant}/v2.0) URLs, accept both if tenant matches
• _validate_scopes(): skip openid, offline_access, profile, email, and any scope ending with /.default

🧪 Verification

Check	Command	Status
Lint suite	make lint	✅
Unit tests	make test	✅
Coverage ≥ 80 %	make coverage	✅
Manual regression	Azure AD gateway fetch tools works	✅

📐 MCP Compliance

• No breaking change to MCP clients
• No change to MCP protocol handling

✅ Checklist

• DCO sign-off present
• Single file changed (token_validation_service.py)
• Backward compatible — only relaxes false-positive blocking, does not weaken legitimate validation

[ecthelion77]

Suggested labels: bug, SHOULD, python, security