fix(core): consume response body in _flush() and _capture() to prevent Cloudflare Workers warnings

`_flush()` and the single-event `_capture()` path both call
`fetchWithRetry()` but discard the returned response without reading
the body. In runtimes like Cloudflare Workers that enforce response
body consumption, this causes cross-request promise resolution warnings
and may silently cancel post-flush error handling continuations.

Consume the response body with `res.text().catch(() => {})` at both
call sites on the success path. Also consume the response body held by
`PostHogFetchHttpError` on error paths (e.g. 413 batch-size reduction)
to prevent the same leak on failed requests.

The `fetchWithRetry()` return type is preserved so callers that need
the response (e.g. surveys API) can still read it.

Fixes #3173

Author: jgarrison929

packages/core/src/posthog-core-stateless.ts

@@ -964,8 +964,16 @@ export abstract class PostHogCoreStateless {
     }
 
     try {
-      await this.fetchWithRetry(url, fetchOptions)
+      const res = await this.fetchWithRetry(url, fetchOptions)
+      // Consume the response body to prevent resource leaks in runtimes like
+      // Cloudflare Workers that require the body to be read before the handler completes.
+      // See: https://github.com/PostHog/posthog-js/issues/3173
+      await res.text().catch(() => {})
     } catch (err) {
+      // Consume the error response body to prevent resource leaks (see #3173)
+      if (err instanceof PostHogFetchHttpError) {
+        await err.response.text().catch(() => {})
+      }
       this._events.emit('error', err)
     }
   }
@@ -1139,8 +1147,17 @@ export abstract class PostHogCoreStateless {
       }
 
       try {
-        await this.fetchWithRetry(url, fetchOptions, retryOptions)
+        const res = await this.fetchWithRetry(url, fetchOptions, retryOptions)
+        // Consume the response body to prevent resource leaks in runtimes like
+        // Cloudflare Workers that require the body to be read before the handler completes.
+        // See: https://github.com/PostHog/posthog-js/issues/3173
+        await res.text().catch(() => {})
       } catch (err) {
+        // Consume the error response body to prevent resource leaks (see #3173)
+        if (err instanceof PostHogFetchHttpError) {
+          await err.response.text().catch(() => {})
+        }
+
         if (isPostHogFetchContentTooLargeError(err) && batchMessages.length > 1) {
           // if we get a 413 error, we want to reduce the batch size and try again
           this.maxBatchSize = Math.max(1, Math.floor(batchMessages.length / 2))