feat(finance): add Decimal support with precision control

[rodrigobnogueira]

Summary

Add optional Decimal return type for price() and price_in_btc() methods to support high-precision financial calculations.

This implementation uses native Decimal generation via integer arithmetic — no intermediate float representation that could introduce precision errors.

Closes #1575

Changes

• Add as_decimal: bool = False parameter for backward compatibility
• Add precision: int parameter to control decimal places (default 2 for price, 8 for BTC)
• Use native Decimal generation (integer arithmetic, no intermediate float)
• Add comprehensive tests for new functionality

Example Usage

from mimesis import Finance
from decimal import Decimal

f = Finance()

# Default float (backward compatible)
f.price()  # 1234.56

# High-precision Decimal
f.price(as_decimal=True)  # Decimal('1234.56')

# Custom precision (e.g., 4 decimals for forex)
f.price(precision=4, as_decimal=True)  # Decimal('1234.5678')

# BTC with satoshi precision (8 decimals)
f.price_in_btc(as_decimal=True)  # Decimal('0.12345678')

Testing

• All 8753 existing tests pass
• Added new tests for Decimal return type and precision control
• Verified seeded reproducibility for Decimal outputs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.91%. Comparing base (64923e6) to head (6a53f6e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1692      +/-   ##
==========================================
+ Coverage   98.90%   98.91%   +0.01%     
==========================================
  Files          50       50              
  Lines        2098     2119      +21     
==========================================
+ Hits         2075     2096      +21     
  Misses         23       23              

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