fix: add .unwrap() to rates.set() calls in doc examples (issue #180 fix 5)

[Copilot]

Fixes No.5 from issue #180.

Problem

ExchangeRates::set returns Result<(), MoneyError>, but several doc examples called it without handling the result:

rates.set(EUR::CODE, dec!(0.8));   // Result ignored
rates.set(IDR::CODE, 17_000);      // Result ignored

This inconsistency (compared to the set method's own doc example which correctly used .unwrap()) teaches users that the Result can be silently discarded, hiding potential OverflowErrors.

Fix

Added .unwrap() to all rates.set(...) calls in doc examples that previously ignored the returned Result:

• Exchange::convert doc example (lines 66–67)
• ExchangeRates struct-level doc example (lines 224–227)
• ExchangeRates::get doc example (lines 413–417)
• ExchangeRates::get_pair doc example (lines 434–438)

Verification

• make all — all 1502 tests pass, doc tests pass
• make lcov — coverage report generated successfully

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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