feat(wallet): implement wallet resource methods

[Zikrullah001]

What Changed

• packages/wallet/src/index.ts: Added getWallet, listWallets, getWalletBalances, and fundWallet resource methods using the shared client helpers, with optional request options, typed parameters, and TSDoc usage examples.
• packages/types/src/dto.ts: Added WalletBalanceQuery and FundWalletInput, with exact decimal-string amounts and typed Stellar asset/issuer fields; used the public-key type for imported wallet addresses.
• packages/types/src/entities.ts: Added StellarAssetCode and StellarPublicKey aliases and applied them to wallet and balance DTO fields while retaining balance amounts as DecimalString.
• packages/wallet/__tests__/wallet.test.ts: Added mocked-client tests for each requested method, including route/query handling and preservation of decimal amounts as strings.

Why

Expose the wallet collection and balance/funding endpoints through the SDK's typed WalletResource API, while preventing monetary precision loss and clarifying Stellar asset and issuer fields.

How it Was Tested

• pnpm --filter @astroid/wallet test: passed, 53 tests across 7 files. New cases cover fetching, listing, balance filters, and funding request payloads with exact decimal strings.
• pnpm --filter @astroid/wallet typecheck: passed.
• pnpm typecheck: currently fails in the fetched upstream baseline at packages/agent/src/__tests__/validation.test.ts, which imports missing CreateAgentDtoSchema and UpdateAgentDtoSchema exports from @astroid/types. This failure is unrelated to the wallet changes.

Closes #275