gamma MarketsRequest: add native active filter support

[morluto]

Summary

gamma::types::request::MarketsRequest currently has no active field/builder method (at least in polymarket-client-sdk 0.4.2).

Because of this, consumers that expose an --active filter (like polymarket-cli) must fetch pages and apply active filtering client-side.

Why this matters

Without native active filtering in the request object:

• API calls cannot express the full query intent (active + other filters) server-side.
• Clients may return undersized results for page-limited requests unless they implement backfill/pagination loops.
• Clients perform extra requests/work that should be avoidable if the backend supports active for markets.

In polymarket-cli, this required additional logic to backfill when --active is set so --limit semantics are preserved.

Current state (SDK)

MarketsRequest includes closed but not active:

• polymarket-client-sdk/src/gamma/types/request.rs

Proposal

If the Gamma markets endpoint supports it, add native active support to MarketsRequest:

• add pub active: Option<bool> to the request type
• expose builder methods (.active(...) / .maybe_active(...))
• ensure serialization/query mapping aligns with the API parameter name
• document in README/examples for Gamma markets filtering

Consumer impact

This would let downstream clients issue precise server-side queries and avoid client-side backfill loops for active filtering.

Context

Opened from follow-up work in polymarket-cli PR:

• fix(cli): correct market price labeling and status/comment filters polymarket-cli#10