fix: use exact string amount for Solana balance instead of float round trip

[samarthpatel24]

Summary

• get_balance was using ui_amount (f64) from the Solana RPC and multiplying back to atomic units, causing floating-point precision loss (e.g. 0.1 * 1_000_000 → 99999 instead of 100000)
• Now uses token_balance.amount (the exact atomic units string from the RPC) directly, matching the pattern already used by the EVM and Tempo providers

Test plan

• cargo check passes
• Existing Solana tests pass
• No ripple effect — NetworkBalance.balance_atomic is already a String field

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