Fixes a small design oversight when creating the SourcegraphModelConfig.

The SourcegraphModelConfig type contains all the configuration knobs for a Sourcegraph admin to control how the "Sourcegraph-supplied LLM models" should be used. For example:

• If set to null, then Sourcegraph will not provide any LLM models for the instance. And only the admin-supplied providers and models will be used.
• It can be configured to poll Cody Gateway to automatically include "new models" so they can be made available to users of the instance as soon as they are served from Cody Gateway.

However, there's something that is lost in translation between "site admin configuring how LLM models get picked up" and "generating the types.ModelConfiguration data structure needed at runtime".

In order to actually use Cody Gateway at runtime, the Completions Provider needs to know the URL endpoint and access token for Cody Gateway. This is made available via the types.Provider.ServerSideConfig.SourcegraphProvider field. (So when creating the "google" completions provider, that's how we know to send the request to Cody Gateway, etc.)

But with the data type we have right now, this data is missing.

With the current way you can configure Cody Enterprise, the accessToken field is optional. Because when we load the configuration, the Sourcegraph instance will automatically create a new access token based on the license key.

But as we transition to the new-style configuration data, we will no longer be able to rely on CompletionsConfig.AccessToken.

So specifically:

1. We need to add this field to the types.SourcegraphModelConfig config data type, so that when we are building the types.ModelConfiguration we know how to create the provider information to contact Cody Gateway as needed.
2. We can make this optional or remove it completely in how the Sourcegraph admin "enters/encodes" the types.SourcegraphModelConfig in their Site configuration data. (And continue to just generate a token based on the license key.)

// conf/computed.go
func getSourcegraphProviderAccessToken(accessToken string, config schema.SiteConfiguration) string {
	// If an access token is configured, use it.
	if accessToken != "" {
		return accessToken
	}

	// Otherwise, use the current license key to compute an access token.
	if config.LicenseKey == "" {
		return ""
	}

	return license.GenerateLicenseKeyBasedAccessToken(config.LicenseKey)
}

Test plan

NA

Changelog

NA