Add asyncio support to the Python SDK

[kvz]

Why

Issue #9 asks for a Pythonic asyncio surface without changing sync callers. This PR adds a separate AsyncTransloadit API on top of the v2 runtime/security branch (#56) so async users can opt in without affecting the existing synchronous client.

This branch is also the first SDK companion for the API2 contract generator work in https://github.com/transloadit/api2/pull/7928. Low-level endpoint methods that the current Python transport can honestly express are generated from API2 endpoint contracts and verified byte-for-byte from API2.

What changed

• Added transloadit.async_client.AsyncTransloadit with async versions of the existing client helpers.
• Added async request/assembly/template helpers and a shared Response wrapper that works for both sync and async responses.
• Added async context manager cleanup plus explicit aclose()/close().
• Kept the sync Transloadit API compatible with existing callers.
• Added generated low-level sync and async endpoint methods for the non-blocked API2 contract operations: Assembly create/replay/notifications, Template create/builtin/full helpers, priority job slot listing, and Template Credential CRUD/type/OAuth validation helpers. These methods live inside marked api2-generated-endpoints blocks so API2 can rewrite them directly.
• Regenerated the endpoint blocks after API2 removed its separate Python SDK plan/snippet file. The generator now reads endpoint contracts directly, including endpoint-local compatibility hints for existing Python method names.
• Hardened sync Assembly edge cases found by council review: non-resumable retries rewind seekable files or reject non-seekable streams, wait/resumable paths reject unexpected non-JSON responses when they cannot safely proceed, polling stays pinned to the original Assembly URL, and repeated poll rate limits are capped.
• Made same-service API URL signing compare hostnames case-insensitively.

TUS/resumable tradeoff

• Non-resumable uploads use aiohttp directly.
• Resumable uploads still delegate to the existing tuspy uploader, but through asyncio.to_thread() so the event loop is not blocked. That keeps the implementation straightforward and honest about the sync TUS path.
• TUS protocol endpoints are intentionally not generated as normal low-level methods yet; they need a richer transport surface for custom verbs, headers, binary bodies, and response-header capture.

Testing

• Added high-level async tests for GET/list/cancel/template flows, non-resumable assembly upload, wait=True polling with mocked asyncio.sleep, async context-manager cleanup, and resumable upload delegation.
• Added sync and async tests proving the generated low-level endpoint methods call the existing request primitives with quoted path params and without surfacing auth-only params.
• Added sync Assembly regression tests for retry rewinding, non-seekable retry rejection, non-JSON response handling, poll URL pinning, and capped poll rate limits.
• Added a same-service mixed-case hostname signing regression test.
• Verified locally with:
  • poetry run pytest
  • From API2: api2/bin/cli.ts contracts sdks --no-motd --platform python --include-generated --sdk-root ../python-sdk-asyncio --compare-existing
  • From API2: api2/bin/cli.ts contracts sdks --no-motd --platform python --sdk-root ../python-sdk-asyncio --rewrite-sdk --compare-existing

This PR is intentionally based on #56 and targets base branch py310-v2.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.40387% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.58%. Comparing base (89a52c3) to head (0c4c974).

Files with missing lines	Patch %	Lines
transloadit/assembly.py	87.50%	12 Missing ⚠️
transloadit/async_request.py	95.67%	7 Missing ⚠️
transloadit/api_url.py	92.85%	2 Missing ⚠️
transloadit/response.py	93.93%	2 Missing ⚠️
transloadit/async_assembly.py	99.28%	1 Missing ⚠️
transloadit/client.py	98.64%	1 Missing ⚠️
transloadit/request.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           py310-v2      #57      +/-   ##
============================================
+ Coverage     86.60%   96.58%   +9.98%     
============================================
  Files             7       14       +7     
  Lines           209      849     +640     
  Branches         19      107      +88     
============================================
+ Hits            181      820     +639     
- Misses           28       29       +1     

Flag	Coverage Δ
unittests	96.58% <96.40%> (+9.98%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[kvz]

Setting this back to draft while we expand the feature scope: the v2 PR should also cover the full Transloadit API endpoint surface before we treat it as ready. I added a matching @todo to the 2.0.0 CHANGELOG section so the release notes get updated once the endpoint coverage is complete.