fix: remove duplicate safe_exp function, use exp_safe instead

[MkDev11]

Summary

Resolves #2149

Problem

The codebase has two similar exponentiation functions in math.rs:

1. exp_safe (line 172): Robust implementation with:

   • Input clamping to [-20, 20] range
   • Smart error handling (returns max_value for positive overflow, 0 for negative)
   • Existing test coverage in tests/math.rs

2. safe_exp (line 1597): Simple wrapper that:

   • Has no input clamping
   • Always returns 0 on any error

Solution

• Remove the duplicate safe_exp function from math.rs
• Update run_epoch.rs to use exp_safe instead

Why exp_safe is better

1. Clamps input to prevent overflow in the first place
2. Returns max_value (not 0) when large positive inputs would overflow - more mathematically correct
3. Has existing test coverage

Testing

• cargo check -p pallet-subtensor passes
• Existing exp_safe tests in tests/math.rs provide coverage

[MkDev11]

@gztensor Can you plz check this PR?