Title: Add property test for stream cancellation idempotency

[Emmyt24]

Description
Implement a property test proving stream cancellation events are idempotent.

Requirements and context

• Must be secure, tested, and documented where applicable
• Should stay reviewable and fit the current monorepo structure

Suggested execution

• Create branch: add-property-test-for-stream-cancellation-idempotency
• Keep changes scoped to the issue and reference the task IDs in the PR

Implement changes

• Generate duplicate stream cancellation events
• Run at least 100 iterations and annotate Property 81
• Assert multiple cancellations don't cause errors
• Verify final status is CANCELLED

Test and commit

• Run the most relevant tests for the touched service or UI surface
• Include idempotency guarantees documentation
• Call out edge cases, assumptions, and any follow-up work

Example commit message

test: add stream cancellation idempotency property

Guidelines

• Prefer small, reviewable PRs
• Keep naming and data contracts consistent with the spec docs
• Preserve responsive and accessible behavior for frontend work

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