Foreign provider config path auth never substitutes the API key into the RPC URL

[netrome]

Background

Discovered in https://github.com/near/mpc-private/issues/251

Providers configured with kind: path authentication have their {api_key} placeholder left as a literal string in the URL. The token value is never substituted, causing all HTTP requests to fail.

User Story

As an MPC operator I'd like to be able to configure path auth

Acceptance Criteria

We've fixed so the path auth config works.

Resources & Additional Notes

Note that we should prefer bearer auth for anyone that supports it. Claude seems to be quick to default to path auth. But for example Infura seems to require path auth so we're not getting around it with them.