fix(farming-pool): return typed error before initialization

[ProtonsAndElectrons]

Summary

• Add PoolError::NotInitialized and an initialization guard for farming-pool entrypoints.
• Replace get_admin and get_stake_token storage unwrap() calls with fallible helpers.
• Return typed contract errors from public farming-pool methods and add regression coverage for uninitialized stake and pause calls.

Fixes #34

Validation

• cargo fmt --check -p farming-pool
• cargo test -p farming-pool (52 passed)

I also tried the broader cargo test / cargo test -p factory on Windows, but both were blocked while linking farming_pool.dll with x86_64-w64-mingw32-gcc: export ordinal too large: 75493.