Add helper for converting raw Stellar event fields to the server trade event schema

[Chucks1093]

Summary

The indexer receives raw Soroban event data from the Stellar RPC and must map it to the server's internal trade event schema. This mapping should live in a dedicated helper so it can be unit tested without invoking the RPC and reused across different event parsers.

Scope

• Add parseStellarTradeEvent(rawEvent): TradeEvent helper
• Maps raw Soroban event topic and data fields to: event_type, creator_id, actor_address, amount, price, fee, ledger_sequence, timestamp
• Throws a typed ParseError if any required field is missing or the wrong type
• Add unit tests for a valid buy event, a valid sell event, and a missing required field

Acceptance Criteria

• Valid buy event maps to correct schema fields
• Valid sell event maps to correct schema fields
• Missing required field throws ParseError
• No RPC calls made in tests

ETA: 12 hours

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Coordinate on Telegram

[Danielobito009]

@Danielobito009 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i want to work on this issue, please assign me.

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[Godfrey-Delight]

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

i would love to work on this

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[extolkom]

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

Assign me pls

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[ojuotimi932]

@ojuotimi932 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please assign

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #513)
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