Route non-tagged OpenAI-compatible text prompts to sample_query in OpenRouter endpoints

[Copilot]

Standard text-to-music requests sent to /v1/chat/completions without <prompt>/<lyrics> tags were being interpreted as direct prompt input, which broke expected behavior for OpenAI-compatible plain-text flows (notably remote Acemusic usage). This change aligns no-tag handling with documented behavior by routing plain non-lyrics text into sample-query mode.

• Input parsing fix (both OpenRouter surfaces)

  • Updated no-tag fallback logic in:
    • acestep/openrouter_adapter.py
    • openrouter/openrouter_api_server.py
  • New behavior:
    • lyrics-like text → lyrics
    • non-lyrics text (no XML tags) → sample_query (instead of prompt)

• Regression coverage

  • Added focused unit tests in acestep/openrouter_adapter_test.py for:
    • plain text (no tags) mapping to sample_query
    • tagged <prompt> still mapping to prompt
    • lyrics-shaped text still mapping to lyrics

• Behavioral example

  # before (no tags):
  # "Generate an upbeat pop song..." -> prompt
  
  # after (no tags):
  # "Generate an upbeat pop song..." -> sample_query
  # (lyrics-shaped text still -> lyrics)