…4276)

This PR fixes Cody autocomplete in some situations, fixing
[PRIME-426](https://linear.app/sourcegraph/issue/PRIME-426/autocomplete-completely-broken-in-main-with-and-sometimes-without-the).

Our story begins with this PR:
https://github.com/sourcegraph/sourcegraph/pull/63886. The PR updated
the `CodyLLMConfigurationResolver.Provider` GraphQL endpoint to no
longer return the "provider" that was put into the site configuration,
but to instead return the _display name_ of the provider, or if multiple
were configured the string "various".

```diff
- func (c *codyLLMConfigurationResolver) Provider() string {
- 	return string(c.config.Provider)
- }
+ func (c *codyLLMConfigurationResolver) Provider() string {
+	if len(r.modelconfig.Providers) != 1 {
+		return "various"
+	}
+	return r.modelconfig.Providers[0].DisplayName
+}
```

This change was wrong on several levels, and unfortunately stemmed from
the original author (me) not tracking down and understanding how the
GraphQL endpoint was actually used.

Once we discovered the problem, we quickly rectified this by changing
the behavior to the more correct version of returning the Provider ID of
the code completion model with
https://github.com/sourcegraph/sourcegraph/pull/64165:

```go
func (r *codyLLMConfigurationResolver) Provider() string {
	return string(r.modelconfig.DefaultModels.CodeCompletion.ProviderID())
}
```

However, after some more testing we discovered yet another edge case. We
didn't realize that the provider "sourcegraph" (which is how you
configure Sourcegraph to use Cody Gateway, using the older site config
method) was required in some scenarios on the Cody client.

So the new logic, of returning the Provider ID was incorrect. (Because
we report the _model_ provider, e.g. "anthropic". Not the _API_
provider, e.g. "sourcegraph"/"Cody Gateway".)

With this change, we should just have the behavior we had initially:
returning whatever the admin had configured in the
"completions.provider" section of the site config. If only the newer
modelconfig settings were used, we return "sourcegraph" if using Cody
Gatway. And if not, just return the Provider ID. (Though in some
situations even that will lead to incorrect results, because ultimately
we need to update the client here.)

## Test plan

I wasn't able to test this super-well manually, and am relying on
Stephen and Taras' knowledge of how the client uses this today.

## Changelog

NA