fix(cli): don't crash tos.py monitor/flash on an unbuilt project

[maidang-xing]

Summary

• tos.py monitor and tos.py flash read using.config via plain dict-subscript (using_data["CONFIG_PLATFORM_CHOICE"], etc.). When the project has never been built, using.config doesn't exist yet, parse_config_file() silently returns {}, and the subscript throws an unhandled KeyError (raw Python traceback) instead of a usable message.
• monitor now calls init_using_config(force=False) before reading the config (same call build already makes), so the correct per-board baudrate is derived from app_default.config even without a prior build.
• flash gets the same init_using_config() call, so an unbuilt project now hits the existing "Not found bin file, please use [tos.py build]." message instead of crashing. Also added a friendly guard + .get() fallback for CONFIG_PLATFORM_CHOICE in get_flash_cmd() — the one remaining spot that could still KeyError (e.g. a hand-edited/corrupted using.config).
• Hardened get_configure_baudrate() / check_bin_file() to use .get() with graceful fallback instead of hard subscripting, since they're shared helpers reachable from more than one call site.

Test plan

• tos.py monitor on an unbuilt project (no using.config) now derives the correct board baudrate instead of crashing with KeyError: 'CONFIG_PLATFORM_CHOICE'
• tos.py flash on an unbuilt project (no using.config, no bin) now prints Not found bin file, please use [tos.py build]. instead of crashing
• tos.py flash with using.config present but missing CONFIG_PLATFORM_CHOICE now prints Not found platform, please use [tos.py config]. instead of crashing
• tos.py flash where only using.config was regenerated but the bin file still exists successfully proceeds to the flash step (self-heals)
• python3 -c "import ast; ast.parse(...)" syntax check on both changed files

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