Simplify execution health check logic:
- If executor responds (2xx or 4xx), tool is HEALTHY
- Only mark BROKEN for 5xx errors or network/timeout failures
- Validation errors (URL format, missing fields) mean tool IS working
- Remove brittle pattern matching for specific error messages
The previous approach tried to match specific error patterns to determine
if an error was "acceptable". This was fragile. The new approach:
- Import check: can we load and describe the tool?
- Execution check: did the tool execute at all?
If a tool throws a validation error, it executed successfully - it's
correctly rejecting invalid test input. Only infrastructure failures
(executor down, network timeout) indicate a truly broken tool.
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