Add manual task acknowledgement

[vvanglro]

Why

Some broker backends support explicit message acknowledgement, but taskiq currently only lets workers choose a fixed automatic acknowledgement point: when received, when executed, or when saved.

For some tasks, acknowledgement needs to happen after an application-level durable action is completed, but before the whole task finishes. Examples include committing an external transaction, persisting an idempotency marker, or coordinating with another system where the task code knows the correct acknowledgement boundary better than the worker.

This change adds an explicit manual acknowledgement mode so advanced users can acknowledge from inside the task through Context.ack(), while preserving the existing automatic acknowledgement behavior for all other modes.

Summary

• add manual acknowledgement type
• expose Context.ack(), Context.is_ackable, and Context.is_acked for task-level manual acknowledgement
• keep automatic acknowledgement idempotent through a shared ack controller
• document manual acknowledgement and add receiver/ack parser tests

Compatibility

Existing acknowledgement modes are unchanged. Manual acknowledgement only works for brokers that yield AckableMessage; otherwise Context.ack() raises a clear runtime error.

Tests

• env -u PYTHONPATH -u VIRTUAL_ENV .venv/bin/python -m pytest -q
• env -u PYTHONPATH -u VIRTUAL_ENV .venv/bin/python -m ruff check .
• env -u PYTHONPATH -u VIRTUAL_ENV .venv/bin/python -m black --check .

[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.20%. Comparing base (fe92683) to head (d6af7d6).

Files with missing lines	Patch %	Lines
taskiq/context.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #637      +/-   ##
==========================================
+ Coverage   80.02%   80.20%   +0.18%     
==========================================
  Files          69       69              
  Lines        2548     2577      +29     
==========================================
+ Hits         2039     2067      +28     
- Misses        509      510       +1     

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