Add R2 example

[adewale]

Summary

Adds a Python Workers example for core Cloudflare R2 Workers API operations.

The example demonstrates:

• object upload/download with put(), get(), and head()
• direct request ReadableStream uploads to R2
• direct R2 ReadableStream responses without copying large objects through Python memory
• range reads
• HTTP metadata and custom metadata
• conditional read/write headers
• checksum and storage class options
• listing with prefix, limit, cursor, delimiter, and include
• single-key and batch deletes
• multipart upload create/upload/complete/abort using ?key= so keys with slashes work
• shared large-file fixture routes for future examples

Stack

This PR is stacked on top of cloudflare#79 (16-sync-http-clients). The diff against add-sync-http-clients-example contains only the R2 example changes.

Tests

python3 -m py_compile 17-r2/src/entry.py
uv run pytest tests/test_examples.py::test_17_r2 -q
uv run ruff check 17-r2 tests/test_examples.py

[adewale]

Closing this stacked fork PR. Reopening as a normal upstream PR that notes its dependency on cloudflare#79.