feat(stellar): add formatAssetIdentifier helper for uniform asset representations (closes #441)

[Ranjeet2063]

Summary

Closes #441

Implements formatAssetIdentifier and parseAssetIdentifier in src/utils/stellar.ts to provide uniform string representations for Stellar native assets and credit assets across API responses, logging, and internal payloads.

Key Changes

• Added formatAssetIdentifier(asset, issuer?):
  • Formats native assets (Asset.native(), "native", "XLM", { code: "native" }, { code: "XLM" }) strictly as "native".
  • Formats credit/issued assets (Asset, { code, issuer }, or code + issuer arguments) as "<code>:<issuer>".
  • Enforces strict validation: throws descriptive errors if asset parameter is missing or non-native assets lack an issuing account.
• Added parseAssetIdentifier(identifier):
  • Parses "native" / "XLM" into { code: "XLM", issuer: null }.
  • Parses "<code>:<issuer>" into { code, issuer }.
• Added complete unit test coverage in tests/stellar-asset-formatting.test.ts (18/18 tests passing).

Verification

• npm test -- tests/stellar-asset-formatting.test.ts passed (18 tests passing).
• npm run build completed with 0 TypeScript errors.
• npm run lint passed with 0 errors.

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