test(wallets): add integration tests for holdings 400 on malformed Stellar address (#511)

[KingDavid9999]

closes #511

Add integration tests for holdings endpoint malformed address validation (#511)

Adds integration test coverage for GET /wallets/:address/holdings confirming that malformed Stellar addresses are rejected before any database query is attempted.

What's tested

• Address that is too short returns 400
• Address with wrong prefix returns 400
• Address with invalid characters returns 400
• Error body identifies the address field in all three cases
• fetchWalletHoldings is not called for any invalid input

Notes

• No production code changes, validation was already in place via WalletHoldingsParamsSchema and the controller's early return on parse failure
• Follows the same structure and conventions as wallet-activity.integration.test.ts

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