[Bug]: _populate_auth not working

[madhav165]

🐞 Bug Summary

When calling get_gateway API, appropriate token, username, password as expected to be set in the response. But, they are not set since _populate_auth was not being called.

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🧩 Affected Component

Select the area of the project impacted:

• mcpgateway - API

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🔁 Steps to Reproduce

1. Add gateway with auth
2. Click edit gateway
3. Check response from GET /gateways/<gateway_id> - auth_value is filled, but appropriate auth_token or auth_username or auth_password not set.

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🤔 Expected Behavior

1. auth_token should be set if auth is bearer
2. auth_username and auth_password should be set if auth is basic
3. auth_header_key and auth_header_value should be set if auth is custom headers

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