Bug: Negative TAO amounts cause massive staking values

[OpenSourceSoul]

Problem
StakeParams::new_tao() accepts negative TAO amounts without validation. When a negative f64 is cast to u64, it overflows and produces a massive value (close to u64::MAX).

Example
Calling StakeParams::new_tao(hotkey, 1, -1.0) would attempt to stake ~18 billion TAO instead of rejecting the negative amount.

Fix
PR #7 contains the fix:

• Validates that TAO amount is non-negative
• Returns Result type for proper error handling
• Adds tests for negative amount rejection

See: #7