fix: map uvicorn stderr prefixes to correct log levels

[Chessing234]

Fixes #1292

Summary

• Uvicorn and similar libraries write INFO-level startup messages to stderr, but FastChat's StreamToLogger redirected all stderr output at ERROR level.
• Parse common log level prefixes on redirected stderr lines and emit them at the matching level, while keeping unparsed stderr at ERROR.

Test plan

• Start python -m fastchat.serve.controller and confirm uvicorn startup lines appear as INFO instead of ERROR

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