feat(ads-client): handle different ad types

[Almaju]

• Adds support for three ad types: Image, Spoc (Sponsored Content), and UA Tile
• Introduces type-specific request methods:
  • request_image_ads() — returns single image ads per placement
  • request_spoc_ads() — returns multiple spoc ads per placement (with count parameter)
  • request_ua_tile_ads() — returns single UA tile ads per placement
• Implements an untagged enum (Ad) to deserialize different ad types from JSON responses
• Adds type filtering logic to extract and validate specific ad types from API responses
• Ensures correct deserialization by ordering enum variants (Spoc/UATile before Image) to prevent misclassification
• Adds error handling for unexpected ad types when the API returns a different type than requested
• Updates FFI layer with corresponding MozAdsImage, MozAdsSpoc, and MozAdsUATile structs for the public API

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[luc-lisi]

This is awesome! Thanks so much for this 🎉

You mentioned it in that other PR but I'd just make sure to rename these from ...UATile -> Tile to fit the new schema, but otherwise should be good to go 🚢

[luc-lisi]

Oh actually, I realize we need to update the docs to include these new endpoints. I think we still have request_ads and are missing the one for tiles. But just a minor change and then it should be good to go!