Python Workers: multipart/form-data body parsing fails at ~1 MB (undocumented limit)

# Python Workers: multipart/form-data body parsing fails at ~1 MB (undocumented limit)

## What version(s) of the runtime are you using?

Latest deploy target as of 2026-02-20 (`compatibility_date = "2024-12-06"`, `compatibility_flags = ["python_workers"]`). Also reproducible locally with `wrangler dev` (wrangler 3.x).

## What steps can reproduce the bug?

1. Create a Python Worker that accepts a `multipart/form-data` file upload (e.g., via FastAPI `UploadFile` or raw Starlette multipart parsing).
2. Upload a file **<= 1024 KB** — request reaches the Worker handler normally.
3. Upload a file **>= ~1040 KB** — request fails before any application code executes.

Binary search confirms the threshold is between 1024 KB and 1040 KB.

Minimal repro using `curl`:

```bash
# Create a 1024 KB test file — succeeds (reaches app code, returns 401 auth error as expected)
dd if=/dev/urandom of=/tmp/test_1024k.bin bs=1024 count=1024
curl -s -o /dev/null -w "%{http_code}" \
-F "file=@/tmp/test_1024k.bin;type=application/pdf" \
https:///api/documents
# Returns: 401

# Create a 1040 KB test file — fails at body parsing layer
dd if=/dev/urandom of=/tmp/test_1040k.bin bs=1024 count=1040
curl -s -o /dev/null -w "%{http_code}" \
-F "file=@/tmp/test_1040k.bin;type=application/pdf" \
https:///api/documents
# Returns: 400 — "There was an error parsing the body"
```

### What is the expected behavior?
Per the Workers platform limits documentation (https://developers.cloudflare.com/workers/platform/limits/#request-and-response-limits), the request body size limit is 100 MB on the Free/Pro plan. A ~1 MB multipart upload should succeed and be available to the Worker handler.

### What do you see instead?
A 400 Bad Request response with body {"detail":"There was an error parsing the body"} is returned. The error originates from the multipart body parser layer (likely python-multipart / Starlette running under Pyodide in workerd), not from any application code — the Worker's fetch handler is never invoked.

### Additional context
This only affects Python Workers. JavaScript/TypeScript Workers on the same account and zone handle the same files without issue (the 100 MB platform limit applies as documented).
The wrangler.jsonc configuration has no body-size-related settings, and the configuration reference (https://developers.cloudflare.com/workers/wrangler/configuration/) does not expose one. The limits key only supports cpu_ms and subrequests.
This limitation is not documented anywhere: not on the Limits page (https://developers.cloudflare.com/workers/platform/limits/), the Known Issues page (https://developers.cloudflare.com/workers/platform/known-issues/), or the Python Workers docs (https://developers.cloudflare.com/workers/languages/python/).
Our MAX_UPLOAD_SIZE_MB app-level env var is set to 10 (10 MB), but the validation code is never reached.
This blocks any real-world file upload use case in Python Workers (e.g., PDF processing, image uploads) for files larger than ~1 MB.