fix: make platform/model filter an enum in llmo apis

[aniham]

Summary

Adds LLM model enum validation to the Brand Presence endpoints (filter-dimensions and weeks). The model query parameter is validated against the llm_model enum from mysticat-data-service; invalid values return 400.

Changes

• Enum values (from mysticat-data-service llm_model): chatgpt-paid, chatgpt-free, google-ai-overview, perplexity, google-ai-mode, copilot, gemini, google, microsoft, mistral, anthropic, amazon
• Default model: chatgpt-free (was chatgpt)
• Validation: Invalid model returns 400 with message: Invalid model. Must be one of: ...
• Docs: Updated filter-dimensions and weeks API docs with enum values and error behavior
• Tests: Added returns badRequest when model is invalid for both handlers; updated default model expectations

[github-actions]

This PR will trigger a patch release when merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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