fix(types): OrderBookSummary.tick_size should be TickSize, not string

[MackDing]

Closes #50

Problem

OrderBookSummary.tick_size was typed as string, but the TickSize literal union already exists in the same file:

export type TickSize = "0.1" | "0.01" | "0.001" | "0.0001";

Consumers who want to feed an order book's tick size back into anything that expects TickSize (CreateOrderOptions.tickSize, isTickSizeSmaller, priceValid, the TickSizes map) have to cast or re-validate — which is exactly what #50 reports.

Change

One-line tightening of the type in OrderBookSummary:

- tick_size: string;
+ tick_size: TickSize;

No runtime change, no API contract change. Just restores end-to-end type safety.

Why this is safe

• The API only ever returns one of those four values. If it ever returns something else, that would already be a backend bug.
• Every other surface in this package treats tick size as TickSize; this aligns OrderBookSummary with the rest of the codebase.
• No test changes needed (the value literal still serializes/deserializes as a string at runtime).

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Note

Low Risk
Low risk: this is a TypeScript type tightening only (no runtime/codepath changes) and may only surface compile-time errors for consumers previously treating tick_size as an arbitrary string.

Overview
Updates OrderBookSummary.tick_size in src/types/clob.ts from string to the existing TickSize literal union, aligning order book responses with the rest of the tick-size-typed API surfaces and improving end-to-end type safety for consumers.

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