feat: add skip_result label to skip result backend storage

[vvanglro]

Summary

• Add a skip_result task label so selected tasks can opt out of result backend storage without raising NoResultError.
• Only boolean values are accepted (True / False); missing label means store results as usual; invalid values raise ValueError.
• When skip_result=True, the worker skips result_backend.set_result and post_save middleware hooks, while acknowledgement (including when_saved) still proceeds.
• Document the label in getting started, architecture overview, and result backends docs; extend with_labels typing to include bool / bytes.

Usage

@broker.task(skip_result=True)
async def push_notification(user_id: int) -> None:
    ...


await push_notification.kicker().with_labels(skip_result=True).kiq(user_id=1)

This is complementary to raising NoResultError for runtime decisions.

Test plan

• Added receiver tests for decorator label, kicker label, skip_result=False, invalid value, and when_saved ack behavior
• pytest tests/receiver/ passes locally

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.41%. Comparing base (ae2b788) to head (3d514c1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #656      +/-   ##
==========================================
+ Coverage   81.29%   81.41%   +0.11%     
==========================================
  Files          69       69              
  Lines        2577     2593      +16     
==========================================
+ Hits         2095     2111      +16     
  Misses        482      482              

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