Axios: Fetch Adapter Header Injection via Inherited FormData getHeaders

## Summary Axios contains a guard in the Node HTTP adapter to avoid using an inherited `Object.prototype.getHeaders` as a FormData header source. The fetch adapter calls the shared `resolveConfig()` helper before dispatch, and that helper lacks the same guard. If another vulnerability pollutes `Object.prototype` with FormData-like properties and `getHeaders()`, the fetch adapter can merge attacker-controlled headers into the outbound request. Axios does not create the prototype pollution source. This is a read-side gadget in the fetch adapter configuration path. ## Impact An attacker with a prior same-process prototype-pollution primitive can inject headers into fetch-adapter requests. Depending on the target service, this may affect authorization, metadata-service access, cache behavior, conditional request handling, or other application-specific header logic. Plain objects are blocked by current FormData detection. The confirmed path uses arrays or non-plain class instances whose prototype chain can resolve polluted FormData-like properties. ## Affected Functionality Affected: - Fetch adapter requests. - `resolveConfig()` handling of `utils.isFormData(data)`. - Request bodies that can be spoofed as FormData through inherited `Symbol.toStringTag`, `append`, and `getHeaders`. Not affected: - Node HTTP adapter's later FormData header path, which checks `data.getHeaders !== Object.prototype.getHeaders`. - Plain object request bodies rejected by current `isFormData()` plain-obje