Log cancellation exceptions in the default handler

[w3lld1]

Summary

• I report OperationCanceledException through the default exception handler instead of suppressing its diagnostics.
• I cover both OperationCanceledException and TaskCanceledException with regression tests.
• I preserve the existing exit code of 1.

Validation

• ./build.sh --test
• git diff --check upstream/main...HEAD

Fixes #2808

[w3lld1]

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