fail fast invalid task composition

[acroca]

• Raise ValueError when the same task instance is passed to when_all/when_any more than once, previously a silent, permanent hang.
• Raise ValueError when a still-pending task that belongs to another pending composite is reused, previously a silent hang.
• Document the CompositeTask.__init__ contract: it replays completions into on_child_completed during construction, so subclasses must set their state before super().__init__() and must not reset counters after it.
• Remove the dead _failed_tasks field (written once, never read; failures flow through _exception).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.85%. Comparing base (fce592c) to head (9e6dcbf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1153      +/-   ##
==========================================
+ Coverage   82.84%   82.85%   +0.01%     
==========================================
  Files         123      123              
  Lines       10121    10127       +6     
==========================================
+ Hits         8385     8391       +6     
  Misses       1736     1736              

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[Copilot]

Pull request overview

This PR improves the Durable Task composition layer in dapr.ext.workflow._durabletask by failing fast on invalid composite construction patterns that previously caused silent, permanent hangs, and by clarifying the CompositeTask.__init__ construction contract.

Changes:

• Add validation to reject duplicate child task instances in when_all/when_any.
• Add validation to reject reusing a pending task that is still owned by another pending composite.
• Add/extend tests covering these fail-fast behaviors and sequential reuse after composite completion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dapr/ext/workflow/_durabletask/task.py	Adds fail-fast validation in CompositeTask.__init__, updates related comments/docs, and removes the unused _failed_tasks field.
tests/ext/workflow/durabletask/test_task.py	Adds unit tests verifying duplicates and overlapping composite ownership are rejected, and that reuse after completion remains allowed.

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[sicoyle]

lgtm - thank you!!