fix(clippy): resolve all clippy errors and warnings

[erning]

Summary

• not_unsafe_ptr_arg_deref (11 errors): Mark all pub extern "C" FFI functions that dereference raw pointers as unsafe, and add # Safety doc sections
• useless_conversion (4 warnings): Remove redundant .into() calls in doubao/qwen websocket sends where the value is already the target type
• needless_borrow (2 warnings): Remove unnecessary & before aggregator.best_text() calls
• needless_borrows_for_generic_args (3 warnings): Remove unnecessary & in contains_key() calls in config migration
• while_let_loop (1 warning): Convert loop { match ... None => break } to idiomatic while let in substitute_env_vars
• too_many_arguments (3 warnings): Add #[allow(clippy::too_many_arguments)] to internal functions (run_session, worker_loop, OpenAiCompatibleProvider::new)

Test plan

• cargo clippy passes with zero errors and zero warnings
• make build succeeds
• Hold-to-talk and tap-to-toggle still work (no behavioral changes — only type-level and lint fixes)