feat: add fetchOrderBooks for multi-market order book retrieval

[psyberck]

Summary (Issue #126)

• Adds new fetchOrderBooks(outcomeIds) API for batch order book retrieval, implemented for Polymarket and Kalshi
• Adds Kalshi GetMarketOrderbooks implicit endpoint
• Renames the ids parameter to outcomeIds across the API surface for consistency with existing unified types.
• Add paramsSerializer so array query params are encoded correctly.

Files of note

• core/src/BaseExchange.ts, core/src/exchanges/kalshi/{index,api,fetcher}.ts, core/src/exchanges/polymarket/{index,fetcher}.ts — new method + refactor
• core/specs/kalshi/Kalshi.yaml, core/src/server/openapi.yaml, docs/api-reference/openapi.json — spec updates
• sdks/{typescript,python}/... — client helper + regenerated reference docs

Side Effects

• Rather than copying the entire openapi.yaml into Kalshi.yaml, only the GetMarketOrderbooks endpoint was added. Upgrading to Kalshi API v3.16.0 introduces extensive interface changes, which would require modifying a large number of existing function signatures.
• The createImplicitMethod function signature was updated to accept parameters of type Record<string, any> | any[] for greater flexibility. A more robust approach exists, but implementing it would necessitate widespread changes across related function calls.
• Aligned indentation and quote style in kalshi/index.ts with project conventions.

Testing

• Ploymarket valid outcome Ids (Python)

- Ploymarket invalid outcome Ids (Python) - Ploymarket valid outcome Ids (TypeScript) - Kalshi valid outcome Ids (Python)

[realfishsam]

Awesome. Will merge asap.

For next time: please copy and paste the code input/output instead of using screenshots! Makes things easier on my end!

[psyberck]

Awesome. Will merge asap.

For next time: please copy and paste the code input/output instead of using screenshots! Makes things easier on my end!

For sure, thanks for pointing out. I will leave my testing scripts in the PR as well in the future. :)