fix: split budget limit error from insufficient credit

[Shedna]

Summary

• the JS SDK treated the platform error code EXCEED_BUDGET_LIMIT the same as INSUFFICIENT_FUNDS_CREDITS, even though the public API exposes them as distinct 429 error conditions
• added LinkupBudgetLimitExceededError and mapped EXCEED_BUDGET_LIMIT to it while keeping the existing insufficient-credit behavior unchanged
• updated the error contract test to assert the new dedicated error class

Validation

• npm run build
• npm run lint
• npm test

[Shedna]

🎉 This PR is included in version 3.2.4 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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