Get agent wallet balance with budget context (HMAC or API key)

<!-- relayer:auth-badge:start --> > **Auth: agent HMAC or API key.** Accepts the agent's own HMAC credentials (runtime self-call) **or** the integrator's ApiKey (backend query on the agent's behalf). <!-- relayer:auth-badge:end --> Returns the agent wallet financial snapshot: address, on-chain USDC balance, budget committed/free amounts, runway estimation, and a Solana explorer URL. Use this from the agent SDK to check fund availability before a payment, or from your backend to monitor agent health. Replaces the deprecated GET /wallet and /wallet-balance.