test: add unit tests for format_stacktrace and MobileErrorHandler

[Dor-bl]

Adds unit test coverage for format_stacktrace and MobileErrorHandler in test/unit/webdriver/errorhandler_test.py.

🎯 What: Missing unit tests for format_stacktrace and MobileErrorHandler.check_response in appium/webdriver/errorhandler.py.
📊 Coverage: Added test cases covering:

• None, empty string, and empty list inputs for format_stacktrace.
• Multi-line string stack traces.
• Frame dictionaries with complete fields (lineNumber, fileName, methodName, className).
• Frame dictionaries with missing fields (defaulting to <anonymous>).
• Sequence containing non-dictionary elements.
• Non-iterable inputs that trigger TypeError.
• MobileErrorHandler.check_response logic including payload parsing, standard errors, custom Appium exception mappings (NoSuchContextException, InvalidSwitchToTargetException), and UnexpectedAlertPresentException.
  ✨ Result: 100% test coverage for appium/webdriver/errorhandler.py.

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PR created automatically by Jules for task 12118612994822022523 started by @Dor-bl

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[Copilot]

Copilot review overview

🔵 Needs a closer look

Add exact-type assertions for the fallback and NoSuchContextException mappings.

Review effort: Lite
Findings: None

What changed in this PR

Adds unit tests for format_stacktrace and MobileErrorHandler.check_response.

Changes:

• Covers stacktrace formatting and edge cases.
• Tests standard, custom, alert, malformed, and fallback error handling.

File	Description
test/​unit/​webdriver/​errorhandler_test.py	Adds comprehensive error-handler unit-test coverage.

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