Feature: Add getAccountInfo() and expireAccount() to StellarService (#79)

[BigBen-7]

Summary

Implements two foundational methods in StellarService to close issues #79 and #80.

Closes #79
Closes #80

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getAccountInfo(contractId) — Issue #80

• Calls get_info() on the EphemeralAccount contract using simulateTransaction (read-only, no signing or fees)
• Returns a typed AccountInfo interface mirroring the Rust AccountInfo struct from bridgelet-core
• Throws BadRequestException if the contract is not initialized
• Throws InternalServerErrorException on RPC failures
• Fully unit-testable without a deployed contract via mocked Soroban RPC responses

expireAccount(contractId) — Issue #79

• Calls getAccountInfo() first to read expiry_ledger before invoking expire()
• Checks current ledger via getLatestLedger() — returns early (non-fatal) if account is not yet expired (scheduling race condition)
• Handles Error::NotExpired from the contract as a non-fatal race condition
• Handles Error::InvalidStatus as terminal — throws BadRequestException with a clear message
• Docblock notes that fund recovery depends on the token transfer stub in bridgelet-core

Tests

11 new unit tests in stellar.service.spec.ts covering both methods, all passing.

[drips-wave]

@BigBen-7 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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