fix(ios): review adjustments for #419

[jkmassel]

Suggested changes from reviewing #419.

Changes

1. Use Int64 for HTTPRequestParser.bytesWritten

bytesWritten was Int while expectedContentLength and maxBodySize are already Int64. This is consistent on all Apple platforms today (where Int is 64-bit), but using Int64 explicitly matches the convention used throughout the parser and avoids unnecessary Int64(...) casts at comparison sites.

2. Add end-to-end test for 413 + CORS headers

The core motivation for the drain+handler routing in #419 is that 413 responses include CORS headers so the WebView's fetch() can read them. This adds an integration test that sends an oversized request to MediaUploadServer and verifies both the 413 status code and the Access-Control-Allow-Origin header.

To support this, MediaUploadServer.start() now exposes a maxRequestBodySize parameter (defaulting to the existing 4 GB), so the test can use a small limit without sending gigabytes of data.

Related issues

• iOS: OutputStream.write return value 0 treated as fatal in bound stream writers #437 — pre-existing: OutputStream.write return value 0 treated as fatal in bound stream writers
• iOS: bound stream writer threads silently swallow errors #438 — pre-existing: bound stream writer threads silently swallow errors
• iOS: MediaUploadServer tests silently skip when server can't bind #439 — pre-existing: MediaUploadServer tests silently skip when server can't bind

[jkmassel]

Recreating with correct base — branch had extra commits from trunk.