Telegram on iOS auto-converts double hyphens (--) to em dashes (—)
or en dashes (–) via autocorrect. This breaks /model flag parsing
since parse_model_flags() only recognizes literal '--provider' and
'--global'.
When the flag isn't parsed, the entire string (e.g. 'glm-5.1 —provider zai')
gets treated as the model name and fails with 'Model names cannot
contain spaces.'
Fix: normalize Unicode dashes (U+2012-U+2015) to '--' when they
appear before flag keywords (provider, global), before flag extraction.
The existing test suite in test_model_switch_provider_routing.py
already covers all four dash variants — this commit adds the code
that makes them pass.
