Add extended support for customizing custom HTTP Headers sent to custom Model Providers

Add support for reading session variables that can be read from configuration. To be specific, I want to be able to configure my custom Model Provider to always send the HTTP header x-opencode-session with a unique-per-session value. In the current setup, I can only specify a hard code version

The current UI for Model Providers in Settings looks like this:

I want to be able to read out a session ID variable from this dialog so I can send a session ID to OpenCode. This is something that OpenCode will start to require from the 6th of September 2026. Failure to do so will result in failed requests

I'm hoping for a simple solution that allows me to read out system variables using ${{ session.id }} that I can use in the Custom Headers (JSON) field. Something like

{ "x-opencode-session": "${{ session.id }}" }

No source file, test, or entry point is named in the issue. Start by locating the Model Providers settings and the code that sends custom headers, then trace how session identifiers are available; done means the documented ${{ session.id }} value resolves to a unique session value in outgoing custom-provider headers.