action: strip Content-Type on body-less apiFetch requests (#692) (#694)

* action: strip Content-Type on body-less apiFetch requests (#692)

Vercel's Next.js lambda adapter (Next 16.1.x) attempts to decode a
request body when Content-Type is set and throws
`SyntaxError: Unexpected end of data` before delegating to the route
handler, returning a 500. Hit /run-context exclusively because it was
the only body-less GET that sent `Content-Type: application/json`.

- Drop `Content-Type: application/json` from the GET in
  `action/utils/runContext.ts` (meaningless on a body-less request).
- Defensively strip any `content-type` header in `action/utils/apiFetch.ts`
  when no body is present so future callers can't reintroduce this.

* apiFetch: soften comment — empirical observation, RFC 9110 §8.3 framing

utils/runContext.ts

@@ -88,7 +88,6 @@ export async function fetchRunContext(params: {
   try {
     const headers: Record<string, string> = {
       Authorization: `Bearer ${params.token}`,
-      "Content-Type": "application/json",
     };
     if (params.oidcToken) {
       headers["X-GitHub-OIDC-Token"] = params.oidcToken;