Fix properties dict mutation in testing.create_context

[gmr]

Summary

Fixes a bug where testing.AsyncTestCase.create_context mutated the caller-supplied properties dict.

Problem / Solution

• testing.AsyncTestCase.create_context mutates the caller-supplied properties dict #98 create_context used properties = properties or {} (keeping the caller's object) then wrote defaults via setdefault, so a caller reusing a non-empty dict across process_message calls got stale content_type/type/timestamp → copy-on-entry (dict(properties) if properties else {}).

Testing

Full suite green (209 tests) + ruff clean. New test in tests/test_testing.py reproduces the mutation (verified failing before, passing after).

Closes #98

Summary by CodeRabbit

• Bug Fixes
  • Improved context creation so provided property dictionaries are no longer modified in place.
  • Reusing the same properties input across multiple context creations now keeps the original values intact, while each context still gets the correct content type.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 74b58a9d-cdf1-43b3-8d3f-f3b8a0b99ac6

📥 Commits

Reviewing files that changed from the base of the PR and between 401dd78 and a9a7353.

📒 Files selected for processing (2)

• rejected/testing.py
• tests/test_testing.py

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📝 Walkthrough

Walkthrough

AsyncTestCase.create_context in rejected/testing.py now copies the caller-supplied properties dict into a new dict before applying setdefault mutations, preventing mutation of the caller's original object. A corresponding test verifies this behavior.

Changes

Properties Dict Mutation Fix

Layer / File(s)	Summary
Copy properties before mutation rejected/testing.py	create_context now constructs a new dict from properties when provided, instead of reusing the caller's mapping directly, before applying setdefault defaults.
Regression test for dict mutation tests/test_testing.py	Adds TestPropertiesNotMutated which reuses the same properties dict across two create_context calls and asserts the original dict is unchanged while each context reflects its own content_type.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Poem

A dict once shared got mutated by mistake,
Two contexts fought over the same content_type at stake.
Now copies are made, safe and new,
The caller's props stay pristine and true.
Hop hop hooray, the bug's undone! 🐰

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main fix: preventing properties dict mutation in create_context.
Linked Issues check	✅ Passed	The code copies the caller-supplied properties dict and the regression test confirms the mutation bug is fixed.
Out of Scope Changes check	✅ Passed	The PR stays focused on the reported bug and its test coverage, with no unrelated changes apparent.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

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[gmr]

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