Add Appium profile config loader, example, server integration, and tests

[kliu2python]

Motivation

• Decouple Appium server/capabilities from code to avoid hardcoded servers and secrets.
• Provide named profiles, inheritance, and environment placeholder expansion for flexible local/cloud setups.
• Expose profile-driven tooling so sessions can be started by name and default profiles can be overridden by env vars.

Description

• Add appium_config.py which implements load_profiles, get_profile, placeholder expansion, profile inheritance, validation, and structured error AppiumConfigError.
• Add appium_profiles.example.yaml to demonstrate profiles, extends, and ${ENV} placeholders and add .gitignore entry for appium_profiles.yaml to keep secrets out of VCS.
• Update server.py to integrate profile APIs: add list_appium_profiles, start_appium_session_with_profile, and default-profile helpers that read APPIUM_MCP_DEFAULT_* env vars, and convert several MCP tools to return structured dicts; also introduce LAST_UI_TREE usage and small screenshot/comment tweaks.
• Add unit tests in tests/test_appium_config.py covering basic profile loading, inheritance, and environment placeholder expansion.

Testing

• Ran the new unit test file with python -m unittest tests/test_appium_config.py and it passed.
• No other automated tests were run as part of this change.

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Codex Task