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

Dependency

Depends on #79.

This PR assumes #79 adds 16-sync-http-clients, so this example is numbered 17-r2. Until #79 merges, this PR's diff may include the preceding sync HTTP example commit as part of the branch history.

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