fix(reporter): avoid duplicate FAIL headers on retry (fix #11454)

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Originally proposed by @arjunharshana in #11459. Their description is reproduced below.

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Description

Resolves #11454

When a test fails with retry: 2, printTaskErrors() merges the identical errors but adds the same task to the merged error entry once for each retry. This causes the reporter to print the same FAIL header multiple times, even though the error itself is merged correctly.

This change ensures a task is only added once to each merged error entry.

Added a regression test covering a deterministic failure with retry: 2. The test fails without the fix and passes with it.

AI-assisted - ChatGPT assisted with the investigation and draft. I reviewed and tested the changes.

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Original implementation from #11459 by @arjunharshana