fix(github): honor GITHUB_GRAPHQL_URL for the GraphQL client (#1575)

The REST client honors GITHUB_API_URL, but the GraphQL client derived its
base URL from GITHUB_API_URL as well and ignored the standard
GITHUB_GRAPHQL_URL variable that GitHub Actions provides. On standard GitHub
Enterprise Server this still worked because @octokit/graphql rewrites a
".../api/v3" REST base to ".../api/graphql", but any deployment whose GraphQL
endpoint is not derivable from the REST base (custom proxy, separate host)
sent GraphQL requests to the wrong URL.

Honor GITHUB_GRAPHQL_URL independently and fall back to GITHUB_API_URL when it
is unset, so behavior is unchanged for github.com and standard GHES. A single
trailing "/graphql" is stripped because @octokit/graphql appends its own.

Add wire-level regression tests that run the real client factory in a fresh
process and assert the final request URLs and Authorization headers;
constructor-option assertions are insufficient because @octokit/graphql
rewrites the path after the client is constructed.

test/fixtures/graphql-endpoint-probe.ts

@@ -0,0 +1,41 @@
+// Wire-level probe for the GitHub API client's endpoint routing.
+//
+// `src/github/api/config.ts` reads GITHUB_API_URL / GITHUB_GRAPHQL_URL at module
+// load time, so each endpoint configuration has to be exercised in its own fresh
+// process (the companion test spawns this file once per case with the relevant
+// env vars set). We stub global fetch to capture the FINAL request URL and
+// Authorization header — asserting constructor options is not enough because
+// @octokit/graphql rewrites/append the path (".../api/v3" -> ".../api/graphql",
+// otherwise it appends "/graphql") after the client is constructed.
+import { createOctokit } from "../../src/github/api/client";
+
+type Captured = { url: string; auth: string | null };
+const captured: Captured[] = [];
+
+globalThis.fetch = (async (input: any, init?: any) => {
+  const url: string =
+    typeof input === "string" ? input : (input?.url ?? String(input));
+  const headers = new Headers(init?.headers ?? input?.headers);
+  captured.push({ url, auth: headers.get("authorization") });
+  return new Response(JSON.stringify({ data: {} }), {
+    status: 200,
+    headers: { "content-type": "application/json" },
+  });
+}) as typeof fetch;
+
+const octokits = createOctokit("test-token");
+
+await octokits.graphql(`query { viewer { login } }`);
+const graphql = captured[captured.length - 1]!;
+
+await octokits.rest.request("GET /meta");
+const rest = captured[captured.length - 1]!;
+
+process.stdout.write(
+  JSON.stringify({
+    graphqlUrl: graphql.url,
+    graphqlAuth: graphql.auth,
+    restUrl: rest.url,
+    restAuth: rest.auth,
+  }),
+);