Polymarket activity endpoint should support market_slug and condition_id filters

[DenisCarriere]

Summary

Add filter support for Polymarket activity queries by market_slug or condition_id.

Desired behavior

• GET /v1/polymarket/markets/activity?market_slug=<slug> filters activity to the matching market.
• GET /v1/polymarket/markets/activity?condition_id=<condition_id> filters activity to the matching condition.
• These filters should compose cleanly with existing pagination and supported query params.

Acceptance criteria

• market_slug is accepted and applied as a query filter.
• condition_id is accepted and applied as a query filter.
• Existing filters keep their current behavior.
• Add or update tests/docs as appropriate.