cody-gateway, sourcegraph: disable code completions for OpenAI

[bobheadxi]

Using OpenAI for Cody code completions is currently untested and unsupported. Despite this, we have at least one customer generating very large numbers of OpenAI code completions requests - enough to get us rate limited by OpenAI.

This change:

• Makes Cody Gateway reject OpenAI code completions requests (responding with 501 not implemented)
• Prevents Sourcegraph from generating OpenAI code completion requests

Test plan

CI

[eseliger]

Works for me, but I'll defer to other people tagged for review here to make the call if this is standing in the way of some other efforts.

[bobheadxi]

Beyang indicated disabling is likely the right way to go, but will wait for another approval here 👍

[philipp-spiess]

I think this makes sense, yeah.

Should we add to the docs that enabling OpenAI disables code completions completely? Just so that people aren't confused.

[github-actions]

The backport to 5.1 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-5.1 5.1
# Navigate to the new working tree
cd .worktrees/backport-5.1
# Create a new branch
git switch --create backport-55624-to-5.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 151f43ff2b6650d8de83ddc99b5f99e661d39b3e
# Push it to GitHub
git push --set-upstream origin backport-55624-to-5.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-5.1

Then, create a pull request where the base branch is 5.1 and the compare/head branch is backport-55624-to-5.1.