This change makes Cody Gateway always apply a wildcard model allowlist,
irrespective of what the configured model allowlist is for an Enterprise
subscription is in dotcom (see #62909).

The next PR in the stack,
https://github.com/sourcegraph/sourcegraph/pull/62912, makes the GraphQL
queries return similar results, and removes model allowlists from the
subscription management UI.

Closes https://linear.app/sourcegraph/issue/CORE-135

### Context

In https://sourcegraph.slack.com/archives/C05SZB829D0/p1715638980052279
we shared a decision we landed on as part of #62263:

> Ignoring (then removing) per-subscription model allowlists: As part of
the API discussions, we've also surfaced some opportunities for
improvements - to make it easier to roll out new models to Enterprise,
we're not including per-subscription model allowlists in the new API,
and as part of the Cody Gateway migration (by end-of-June), we will
update Cody Gateway to stop enforcing per-subscription model allowlists.
Cody Gateway will still retain a Cody-Gateway-wide model allowlist.
[@chrsmith](https://sourcegraph.slack.com/team/U061QHKUBJ8) is working
on a broader design here and will have more to share on this later.

This means there is one less thing for us to migrate as part of
https://github.com/sourcegraph/sourcegraph/pull/62934, and avoids the
need to add an API field that will be removed shortly post-migration.

As part of this, rolling out new models to Enterprise customers no
longer require additional code/override changes.

## Test plan

Set up Cody Gateway locally as documented, then `sg start dotcom`. Set
up an enterprise subscription + license with a high seat count (for a
high quota), and force a Cody Gateway sync:

```
curl -v -H 'Authorization: bearer sekret' http://localhost:9992/-/actor/sync-all-sources
```

Verify we are using wildcard allowlist:

```sh
$ redis-cli -p 6379 get 'v2:product-subscriptions:v2:slk_...'
"{\"key\":\"slk_...\",\"id\":\"6ad033f4-c6da-43a9-95ef-f653bf59aaac\",\"name\":\"bobheadxi\",\"accessEnabled\":true,\"endpointAccess\":{\"/v1/attribution\":true},\"rateLimits\":{\"chat_completions\":{\"allowedModels\":[\"*\"],\"limit\":660,\"interval\":86400000000000,\"concurrentRequests\":330,\"concurrentRequestsInterval\":10000000000},\"code_completions\":{\"allowedModels\":[\"*\"],\"limit\":66000,\"interval\":86400000000000,\"concurrentRequests\":33000,\"concurrentRequestsInterval\":10000000000},\"embeddings\":{\"allowedModels\":[\"*\"],\"limit\":220000000,\"interval\":86400000000000,\"concurrentRequests\":110000000,\"concurrentRequestsInterval\":10000000000}},\"lastUpdated\":\"2024-05-24T20:28:58.283296Z\"}"
```

Using the local enterprise subscription's access token, we run the QA
test suite:

```sh
$ bazel test --runs_per_test=2 --test_output=all //cmd/cody-gateway/qa:qa_test --test_env=E2E_GATEWAY_ENDPOINT=http://localhost:9992 --test_env=E2E_GATEWAY_TOKEN=$TOKEN
INFO: Analyzed target //cmd/cody-gateway/qa:qa_test (0 packages loaded, 0 targets configured).
INFO: From Testing //cmd/cody-gateway/qa:qa_test (run 1 of 2):
==================== Test output for //cmd/cody-gateway/qa:qa_test (run 1 of 2):
PASS
================================================================================
INFO: From Testing //cmd/cody-gateway/qa:qa_test (run 2 of 2):
==================== Test output for //cmd/cody-gateway/qa:qa_test (run 2 of 2):
PASS
================================================================================
INFO: Found 1 test target...
Target //cmd/cody-gateway/qa:qa_test up-to-date:
  bazel-bin/cmd/cody-gateway/qa/qa_test_/qa_test
Aspect @@rules_rust//rust/private:clippy.bzl%rust_clippy_aspect of //cmd/cody-gateway/qa:qa_test up-to-date (nothing to build)
Aspect @@rules_rust//rust/private:rustfmt.bzl%rustfmt_aspect of //cmd/cody-gateway/qa:qa_test up-to-date (nothing to build)
INFO: Elapsed time: 13.653s, Critical Path: 13.38s
INFO: 7 processes: 1 internal, 6 darwin-sandbox.
INFO: Build completed successfully, 7 total actions
//cmd/cody-gateway/qa:qa_test                                            PASSED in 11.7s
  Stats over 2 runs: max = 11.7s, min = 11.7s, avg = 11.7s, dev = 0.0s

Executed 1 out of 1 test: 1 test passes.
```