Fix _auth_populate in GatewayRead

[madhav165]

🐛 Bug-fix PR

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📌 Summary

Fixes the computation of user_name and password in get_gateway function

🐞 Root Cause

1. @classmethod used before @model_validator(mode="after") declaration. This was interfering the calling of the _populate_auth function
2. auth_value was checked for username and password directly instead of within Authorization value.

🧪 Verification

Check	Command	Status
Unit tests	make test	pass

📐 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