fix: align mUSD decimals to 6 in default token metadata

[salimtb]

Set the hardcoded mUSD token metadata to 6 decimals in both assets controllers so seeded defaults match the token contract and render balances correctly.

Explanation

References

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
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Low risk: changes only hardcoded default mUSD metadata/seed values (decimals) and updates/extends unit tests and changelogs; no runtime control flow or security-sensitive logic is altered.

Overview
Fixes the default/seeded MetaMask USD (mUSD) token metadata to use 6 decimals instead of 18 across both @metamask/assets-controller (default tracked asset metadata) and @metamask/assets-controllers (TokensController/TokenDetectionController seeding).

Adds/updates tests to assert the seeded mUSD token is added with decimals: 6, and records the fix in both package changelogs.

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