EW-6888 Report killSwitch exceptions as failed

[fhanau]

These errors don't actually indicate that the runtime is overloaded, assign the default exception type to them.

See the downstream PR for context.

[ask-bonk]

The exploration is thorough and confirms the change is safe. Let me verify the key points:

1. Exception detail-based routing: outcomeFromException() in observer.c++ checks SCRIPT_KILLED_DETAIL_ID before checking the exception type, so the EventOutcome::KILL_SWITCH mapping is unaffected by this change.

2. HTTP status code impact: In worker-entrypoint.c++ and server.c++, OVERLOADED maps to HTTP 503 while other types map to 500. Changing from Overloaded to Failed means kill switch errors will now produce 500 instead of 503 — which is the stated intent ("These errors don't actually indicate that the runtime is overloaded").

3. JS .overloaded property: Kill switch exceptions converted to JS will no longer have .overloaded = true set — again, correct per the PR's intent.

4. Tests are updated: Both test cases in test.c++ are updated to assert FAILED instead of OVERLOADED.

This is a clean, well-scoped change. The code is correct, the tests cover the modified behavior, and no backward compatibility concerns exist (this is internal exception type classification, not a user-facing API change).

LGTM

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