fix: guard error handler against non-object thrown values

[AI-Reviewer-QS]

Summary

• The in operator throws a TypeError when its right-hand operand is not an object
• In the tool execution error handler, both 'message' in err and 'cause' in err crash if the caught value is a primitive (null, string, number, undefined)
• Added typeof err === 'object' checks before using the in operator to ensure primitive thrown values are safely converted to strings

Reproduction

try {
  throw null; // or throw "some string", throw 42
} catch (err) {
  'message' in err; // TypeError: Cannot use 'in' operator to search for 'message' in null
}

Test plan

• Verify tool error handling with throw null
• Verify tool error handling with throw "string"
• Verify tool error handling with throw 42
• Verify tool error handling with standard throw new Error("msg") still works
• Verify error with cause property still includes cause message

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[OrKoN]

Is there a scenario where a non Error object is thrown?

[Lightning00Blade]

We can easily enforce this with '@typescript-eslint/only-throw-error': 'error', rather then having more code handling.

[OrKoN]

We can easily enforce this with '@typescript-eslint/only-throw-error': 'error', rather then having more code handling.

would not help for errors thrown by dependencies

[OrKoN]

I think this is redundant for now, please let us know if you have a scenario where a non-object is thrown.