fix: Disable CA Cert validation on authless MCPs

[gcgoncalves]

🐛 Bug-fix PR

📌 Summary

Closes #5070

🔁 Reproduction Steps

UI Reproduction Steps

1. Access the admin UI
2. Navigate to the Gateways tab
3. Fill the MCP form with a valid MCP server, with authentication type=none and no CA certificate
4. Submit the form
5. Verify the form validation errors

API Reproduction Steps

1. Generate a valid JWT token for authentication:
   export TOKEN=$(python -m mcpgateway.utils.create_jwt_token --username admin@example.com --exp 10080 --secret your-secret-key)

2. Attempt to register an authless MCP server via POST /gateways:

curl -X POST http://localhost:4444/gateways \
 -H "Content-Type: application/json" \
 -H "Authorization: Bearer $TOKEN" \
 -d '{
   "name": "test-authless-server",
   "url": "http://localhost:9000",
   "auth_type": "none"
 }'

3. Observe the validation error response:
   {"detail":[{"type":"value_error","loc":["body","auth_value"],"msg":"Value error, Invalid 'auth_type'. Must be one of: basic, bearer, oauth, authheaders, or query_param.","input":null,"ctx":{"error":{}}}]}%

4. Confirm the same error occurs with explicit null:

# With explicit null
curl -X POST http://localhost:8000/gateways \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '{
    "name": "test-authless-server",
    "url": "http://localhost:9000",
    "auth_type": "none",
    "auth_query_param_key": null
  }'

5. Observe the validation error response:

{"detail":[{"type":"value_error","loc":["body","auth_value"],"msg":"Value error, Invalid 'auth_type'. Must be one of: basic, bearer, oauth, authheaders, or query_param.","input":null,"ctx":{"error":{}}}]}%

6. Confirm the validation error occurs with an empty string:

# With empty string
curl -X POST http://localhost:8000/gateways \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '{
    "name": "test-authless-server",
    "url": "http://localhost:9000",
    "auth_type": "none",
    "auth_query_param_key": ""
  }'

7. Observe the validation error response:

{"detail":[{"type":"string_pattern_mismatch","loc":["body","auth_query_param_key"],"msg":"String should match pattern '^[a-zA-Z_][a-zA-Z0-9_\\-]*$'","input":"","ctx":{"pattern":"^[a-zA-Z_][a-zA-Z0-9_\\-]*$"}},{"type":"value_error","loc":["body","auth_value"],"msg":"Value error, Invalid 'auth_type'. Must be one of: basic, bearer, oauth, authheaders, or query_param.","input":null,"ctx":{"error":{}}}]}%

🐞 Root Cause

There is a mismatch between the UI form and backend validation: the admin UI's auth_type dropdown sent an empty string value="" for the "None" option, but the Pydantic schemas expected either a valid auth type string or Python None. The schemas lacked a validator to normalize the string "none" to Python None, causing validation failures when users selected "No authentication" in the UI.

💡 Fix Description

This PR solves the issue by adding a Pydantic @field_validator to both GatewayCreate and GatewayUpdate schemas that normalizes empty strings and the literal string "none" (case-insensitive) to Python None, ensuring the backend correctly interprets the UI's "No authentication" selection. The fix includes comprehensive test coverage for all auth_type variations (valid types, None, empty string, "none" string) across both create and update operations.

🧪 Verification

Check	Command	Status
Lint suite	make lint
Unit tests	make test
Coverage ≥ 80 %	make coverage
Manual regression no longer fails	steps / screenshots

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed