Add helper for safely serializing BigInt values in JSON API responses

[Chucks1093]

Summary

XLM amounts and ledger sequences are stored as BigInt in JavaScript but JSON.stringify throws on BigInt values by default. A shared serialization helper should convert BigInt fields to strings before they reach the response layer so the app never throws a silent serialization error.

Scope

• Add a serializeBigInt(value: unknown): unknown helper that recursively converts BigInt values to strings in objects and arrays
• Use the helper in the response serializer for all endpoints that return price or amount fields
• Add unit tests for: top-level BigInt, nested BigInt in object, BigInt in array, non-BigInt value unchanged

Acceptance Criteria

• Top-level BigInt converts to string
• Nested BigInt in an object converts correctly
• BigInt inside an array converts correctly
• Non-BigInt values pass through unchanged

ETA: 12 hours

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Coordinate on Telegram

[Godfrey-Delight]

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Hi maintainer, I would love to work on this issue

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[Gutopro]

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Can this be assigned to me

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[emmanueltony792-blip]

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i am capable of working on this issue perfectly

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[Jayydy]

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Hello, can i be assigned to this issue?

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[cyberdocs120]

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I can work on this, please assign me

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[deep-bhikadiya]

@deep-bhikadiya has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, this looks like something I can help with. I'd appreciate being assigned to it.

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[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #485)
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