chore: update stablecoins LD config format

[micaelae]

Explanation

References

Fixes https://github.com/MetaMask/core/pull/6616

Checklist

• [ ] I've updated the test suite for new or updated code as appropriate
• [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• [ ] I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• [ ] I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

---

[!NOTE] Moves stablecoin config to a top-level CAIP asset list and updates slippage, selectors, validators, and CAIP formatting accordingly.

• Feature flags/config:
  • Add stablecoins: CaipAssetType[] to platform config and default config; remove chain-level stablecoins.
  • Update PlatformConfigSchema to validate top-level CAIP stablecoins list.
• Slippage:
  • Refactor getDefaultSlippagePercentage to take srcAssetId, destAssetId, and global stablecoins (CAIP); adjust logic for cross-chain, Solana, and EVM stablecoin pairs.
  • Export getDefaultSlippagePercentage and BRIDGE_DEFAULT_SLIPPAGE from index.
• Selectors/exports:
  • Remove selectDefaultSlippagePercentage selector and its exports/usages.
• CAIP formatters:
  • formatAddressToAssetId now always returns a CAIP asset ID; lowercases EVM addresses and uses erc20: vs non‑EVM token:.
  • Improve error messages and deprecate formatAddressToCaipReference in JSDoc; add explicit throws docs.
• Tests:
  • Update slippage tests to use CAIP asset IDs and new getDefaultSlippagePercentage API.
  • Adjust expectations per updated schemas and utilities.

^{Written by Cursor Bugbot for commit 80f273579c7ac8352526af1a3232ae579dbbc076. This will update automatically on new commits. Configure here.}