feat: adds deeply liquid stablecoin slippage value of 0.5

[ghgoodreau]

Description

Purpose:

To automatically apply a lower default slippage (0.5%) when fetching swap quotes for stablecoin pairs (like USDC/USDT), while using the user's custom setting for other pairs. This improves the swap experience for stablecoins and avoids unnecessary API calls with incorrect slippage.

Changes Made:

• Address-Based Stablecoin Definition: Introduced a StablecoinsByChainId map in shared/constants/swaps.ts that lists known stablecoin contract addresses for each supported network, mirroring the mobile implementation.
• Refactored Slippage Logic: Modified the fetchQuotesAndSetQuoteState function in ui/ducks/swaps/swaps.js to:
  --- Check the from and to token addresses against the StablecoinsByChainId map for the current network using a case-insensitive comparison.
  --- Determine the correct slippage (Slippage.stable or the user's maxSlippage) before dispatching the quote fetch action.
• Updated Fetch & Analytics: Ensured the determined slippage value is passed to the API request and correctly logged in analytics events.

Related issues

Fixes: https://consensyssoftware.atlassian.net/browse/MMS-2169

Manual testing steps

1. Go to the swap page
2. On any EVM chain, try to swap between USDT <-> USDC
3. Observe a default slippage value of 0.5% instead of 2%

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[metamaskbot]

✨ Files requiring CODEOWNER review ✨

🔄 @MetaMask/swaps-engineers

• ui/pages/swaps/prepare-swap-page/prepare-swap-page.js
• ui/pages/swaps/transaction-settings/transaction-settings.js

[infiniteflower]

Non-blocking comment: Set has O(1) lookups with set.has(...), if we're just iterating through it instead (O(n)), could just use a simple array.

Doesn't really matter because the list is so small though.

[metamaskbot]

Builds ready [4ac9dae]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• bundle size: Bundle Size Stats
• user-actions-benchmark: User Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all CircleCI artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
  • content-script: 0
  • offscreen: 0

UI Startup Metrics (1212 ± 59 ms)

Platform	BuildType	Page	Metric	Mean (ms)	Min (ms)	Max (ms)	Std Dev (ms)	P 75 (ms)	P 95 (ms)
Chrome	Browserify	Home	uiStartup	1212	1109	1429	59	1252	1319
			load	1064	977	1249	54	1139	997
			domContentLoaded	1058	970	1241	54	1143	994
			domInteractive	16	13	45	5	16	27
			firstPaint	798	81	1216	402	232	985
			backgroundConnect	10	6	27	3	9	10
			firstReactRender	20	14	54	7	20	40
			getState	12	5	63	9	7	9
			initialActions	0	0	1	0	0	1
			loadScripts	812	728	986	51	840	906
			setupStore	7	4	14	2	7	8
	Webpack	Home	uiStartup	2145	1736	2596	186	2275	2409
			load	1702	1257	2181	181	1776	2056
			domContentLoaded	1696	1252	2177	179	1773	2034
			domInteractive	17	12	91	12	14	50
			firstPaint	160	64	374	61	248	91
			backgroundConnect	36	11	354	48	35	56
			firstReactRender	154	52	352	107	59	86
			getState	26	4	393	67	7	9
			initialActions	2	1	7	1	3	4
			loadScripts	1685	1243	2172	171	1766	1970
			setupStore	15	6	33	6	6	9
Firefox	Browserify	Home	uiStartup	1378	1155	1700	116	1452	1621
			load	1238	1034	1531	114	1286	1490
			domContentLoaded	1237	1034	1531	114	1286	1490
			domInteractive	105	37	310	40	88	97
			firstPaint	NaN	NaN	NaN	NaN	NaN	NaN
			backgroundConnect	24	16	92	9	24	43
			firstReactRender	22	19	50	4	22	29
			getState	9	4	214	21	7	8
			initialActions	0	0	1	0	0	1
			loadScripts	1215	1018	1497	112	1262	1472
			setupStore	8	4	198	19	6	7
	Webpack	Home	uiStartup	1557	1391	1961	112	1605	1782
			load	1333	1180	1686	102	1381	1571
			domContentLoaded	1332	1180	1686	102	1381	1571
			domInteractive	96	46	189	23	89	97
			firstPaint	NaN	NaN	NaN	NaN	NaN	NaN
			backgroundConnect	27	19	53	8	28	45
			firstReactRender	36	30	64	6	39	46
			getState	12	4	331	32	8	9
			initialActions	1	0	5	1	1	1
			loadScripts	1309	1159	1661	101	1353	1539
			setupStore	8	5	42	4	8	9