fix(predict): order calculations (MetaMask#22900)

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## **Description**

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also include relevant motivation and context. Have in mind the following
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2. What is the improvement/solution?
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We seem to be wrongly calculating the order values. The CLOB client code
seems quite different than ours, and they use `roundDown()` when
rounding the price for market orders, but that somehow works for them
because they use the `worstPrice` and their `marketPrice` and we are
using `avgPrice`.

In our case, we want the `takerAmount` to be the exact `shareAmount` we
would get with the dollar amount we are willing to spend and, since we
have that value from `matchBuyOrder`/`matchSellOrder`, we do **not**
need to recalculate it, as it will lose precision.

This not only simplifies the logic, but also removes unnecessary
calculations which were leading to incorrect approximations.

### Note

I want to continue investigating the differences between the CLOB client
logic and ours and see if we should revert to what they do.

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## **Related issues**

Fixes:

## **Manual testing steps**

```gherkin
Feature: fix order calculations

  Scenario: user wants to place a Buy order of $1 in a liquid market
    Given share price is $0.11

    When user tries to submit the order, we calculate that we get `9.090909090909092` shares
    This leads to an `avgSharePrice = $1 / 9.090909090909092 shares = $0.109999999`
    Then, if we round this number down, we get a share price of `$0.10`, which is much cheaper than it should be
    This will lead to a much higher `takerAmount`, which makes the order fail
```

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## **Pre-merge author checklist**

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Reworks market order preview to compute maker/taker amounts directly
from matched book sizes, removing avgPrice-based rounding and the unused
roundOrderAmounts helper (and its tests).
> 
> - **Predict/Polymarket utils**:
>   - Adjust `previewOrder` to derive amounts from order book matches:
> - BUY: `makerAmount = roundDown(size, roundConfig.size)`; `takerAmount
= roundOrderAmount(shareAmount, roundConfig.amount)`.
> - SELL: `makerAmount = roundDown(size, roundConfig.size)`;
`takerAmount = roundOrderAmount(dollarAmount, roundConfig.amount)`.
>   - Remove `roundOrderAmounts` and dependency on `avgPrice`.
>   - Drop `RoundConfig`-related usage for that helper.
> - **Tests**:
> - Remove `roundOrderAmounts` import and its test suite in
`utils.test.ts`.
> - Keep `previewOrder` tests aligned with the new calculation approach.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
60bd2a3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: matallui

app/components/UI/Predict/providers/polymarket/utils.test.ts

@@ -64,7 +64,6 @@ import {
   roundDown,
   roundUp,
   roundOrderAmount,
-  roundOrderAmounts,
   previewOrder,
   getAllowanceCalls,
 } from './utils';
@@ -2276,62 +2275,6 @@ describe('polymarket utils', () => {
     });
   });
 
-  describe('roundOrderAmounts', () => {
-    const mockRoundConfig = {
-      price: 4,
-      size: 2,
-      amount: 2,
-    };
-
-    it('rounds BUY order amounts correctly', () => {
-      const result = roundOrderAmounts({
-        roundConfig: mockRoundConfig,
-        side: Side.BUY,
-        size: 10.556,
-        price: 0.55555,
-      });
-
-      expect(result.makerAmount).toBe(10.55);
-      expect(result.takerAmount).toBeGreaterThan(0);
-    });
-
-    it('rounds SELL order amounts correctly', () => {
-      const result = roundOrderAmounts({
-        roundConfig: mockRoundConfig,
-        side: Side.SELL,
-        size: 10.556,
-        price: 0.55555,
-      });
-
-      expect(result.makerAmount).toBe(10.55);
-      expect(result.takerAmount).toBeGreaterThan(0);
-    });
-
-    it('handles price rounding down', () => {
-      const result = roundOrderAmounts({
-        roundConfig: mockRoundConfig,
-        side: Side.BUY,
-        size: 100,
-        price: 0.456789,
-      });
-
-      expect(result.makerAmount).toBe(100);
-      expect(result.takerAmount).toBeGreaterThan(0);
-    });
-
-    it('applies additional rounding when necessary', () => {
-      const result = roundOrderAmounts({
-        roundConfig: mockRoundConfig,
-        side: Side.BUY,
-        size: 123.456789,
-        price: 0.123456789,
-      });
-
-      expect(result.makerAmount).toBeLessThanOrEqual(123.46);
-      expect(result.takerAmount).toBeGreaterThan(0);
-    });
-  });
-
   describe('previewOrder', () => {
     beforeEach(() => {
       mockFetch.mockReset();

app/components/UI/Predict/providers/polymarket/utils.ts

@@ -52,7 +52,6 @@ import {
   PolymarketPosition,
   TickSize,
   OrderBook,
-  RoundConfig,
 } from './types';
 import { PREDICT_ERROR_CODES } from '../../constants/errors';
 
@@ -1119,35 +1118,6 @@ export const roundOrderAmount = ({
   return amount;
 };
 
-export const roundOrderAmounts = ({
-  roundConfig,
-  side,
-  size,
-  price,
-}: {
-  roundConfig: RoundConfig;
-  side: Side;
-  size: number;
-  price: number;
-}): { makerAmount: number; takerAmount: number } => {
-  const rawPrice = roundDown(price, roundConfig.price);
-  const rawMakerAmt = roundDown(size, roundConfig.size);
-  let rawTakerAmt;
-  if (side === Side.BUY) {
-    rawTakerAmt = rawMakerAmt / rawPrice;
-  } else {
-    rawTakerAmt = rawMakerAmt * rawPrice;
-  }
-  rawTakerAmt = roundOrderAmount({
-    amount: rawTakerAmt,
-    decimals: roundConfig.amount,
-  });
-  return {
-    makerAmount: rawMakerAmt,
-    takerAmount: rawTakerAmt,
-  };
-};
-
 export const previewOrder = async (
   params: Omit<PreviewOrderParams, 'providerId'>,
 ): Promise<OrderPreview> => {
@@ -1167,12 +1137,10 @@ export const previewOrder = async (
       asks,
       dollarAmount: size,
     });
-    const avgPrice = size / shareAmount;
-    const { makerAmount, takerAmount } = roundOrderAmounts({
-      roundConfig,
-      side,
-      size,
-      price: avgPrice,
+    const makerAmount = roundDown(size, roundConfig.size);
+    const takerAmount = roundOrderAmount({
+      amount: shareAmount,
+      decimals: roundConfig.amount,
     });
     return {
       marketId,
@@ -1201,12 +1169,10 @@ export const previewOrder = async (
     bids,
     shareAmount: size,
   });
-  const avgPrice = dollarAmount / size;
-  const { makerAmount, takerAmount } = roundOrderAmounts({
-    roundConfig,
-    side,
-    size,
-    price: avgPrice,
+  const makerAmount = roundDown(size, roundConfig.size);
+  const takerAmount = roundOrderAmount({
+    amount: dollarAmount,
+    decimals: roundConfig.amount,
   });
   return {
     marketId,