fix: follow-up fixes for conditional VCS requests (#27711)

Follow-ups from #27627 

- Memoizes `Config.Git()` with a mutex so the provider's ETag response
cache survives across calls. Only successful construction is cached;
errors are retried.
- Moves the HTTP client onto `Config.HTTPClient`, wired through
`ConvertConfig`, so `Git()` no longer takes a per-call argument that
would be silently ignored after memoization.
- `newGitHub` and `newGitLab` now return `(Provider, error)`,
eliminating the typed-nil-interface class in `gitprovider.New` rather
than the single instance.
- Gates the 304 branch on a `haveCached` flag instead of a nil body
check.
- Only caches bodies that decode successfully, preventing poisoned
entries.
- Keys the response cache on the full token digest rather than a
truncated prefix.
- Tests added: `TestConfigGitMemoizesProvider`,
`TestConfigGitRetriesOnConstructorError`,
`TestGitLabConstructorErrorReturnsNilInterface`,
`TestResponseCacheStore`,
`TestConditionalRequestReuse/MalformedResponseNotCached`;
`TestConvertYAML/CustomScopesAndEndpoint` now asserts
`Config.HTTPClient` wiring.

Follow-ups tracked in #28139, #28140, #28141, #28142.

> 🤖 Generated by Coder Agents on behalf of @johnstcn.

cli/server.go

@@ -979,6 +979,7 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 				oauthInstrument,
 				mergedExternalAuthProviders,
 				vals.AccessURL.Value(),
+				httpClient,
 			)
 			if err != nil {
 				return xerrors.Errorf("convert external auth config: %w", err)