fix: catch fetch errors in ensureBuilderFeeRateCached for browser compatibility

[osr21]

Problem

@polymarket/clob-client-v2 v1.0.1+ calls GET /fees/builder-fees/<code> inside ensureBuilderFeeRateCached on every createOrder() and createMarketOrder() call. This endpoint returns 404 with no CORS headers, which causes an immediate network/CORS error in any browser environment (Next.js, React, Vite, etc.) — before any order is signed or submitted.

The result: order creation is completely broken in browsers for any user on v1.0.1+, even if they never set a builderCode. This is because the method is also invoked via getBuilderTakerFeeRate on the createMarketOrder path.

Root cause

private async ensureBuilderFeeRateCached(builderCode?: string): Promise<void> {
    if (!builderCode || builderCode === bytes32Zero) return; // ← guards undefined/zero
    if (builderCode in this.builderFeeRates) return;        // ← cache hit

    // ↓ throws CORS error in browsers — endpoint has no Access-Control-Allow-Origin
    const result = await this.get(`${this.host}${GET_BUILDER_FEES}${builderCode}`);
    ...
}

Fix

Wrap the fetch in try/catch. On failure, default to { maker: 0, taker: 0 }. Builder fees are additive — defaulting to zero is safe and means order creation is not gated on a non-critical endpoint.

try {
    const result = await this.get(`${this.host}${GET_BUILDER_FEES}${builderCode}`);
    this.builderFeeRates[builderCode] = {
        maker: result.builder_maker_fee_rate_bps / BUILDER_FEES_BPS,
        taker: result.builder_taker_fee_rate_bps / BUILDER_FEES_BPS,
    };
} catch {
    // Endpoint returns 404 with no CORS headers in browser environments.
    // Default to zero so order creation is not blocked. Fixes #51.
    this.builderFeeRates[builderCode] = { maker: 0, taker: 0 };
}

Closes #51.

---

Note

Low Risk
Small defensive change on a non-critical fee lookup; worst case is underestimating builder fees in the browser when the API is unreachable.

Overview
ensureBuilderFeeRateCached no longer lets a failed GET /fees/builder-fees/<code> abort order flows. The fetch is wrapped in try/catch, and on error the client caches { maker: 0, taker: 0 } so createMarketOrder / buy-balance paths that call this helper can continue when the endpoint errors (e.g. 404 without CORS in the browser).

The catch path only writes that zero fallback if the cache slot is still empty, so a concurrent successful fetch is not overwritten.

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