Add expensetracker sample project with test coverage analysis

[hatcheric950]

$(cat <<'EOF'

Summary

• Creates a small Python expense-tracking library (Account, Transaction, Budget, utils) with pytest + pytest-cov configured
• Adds partial tests that intentionally cover only happy paths, leaving clear gaps for demonstration
• Adds COVERAGE_ANALYSIS.md with a full breakdown of gaps and concrete proposed test cases

Coverage baseline (branch coverage enabled)

Module	Cover	Missing lines
account.py	69%	8, 10, 17, 23, 25, 36, 39
budget.py	19%	8–14, 17–21, 24–26, 29–31, 34
transaction.py	73%	18, 20, 22, 35, 43
utils.py	26%	11–13, 18–25, 32–33, 38–39
TOTAL	49%

Proposed improvements (priority order)

1. budget.py — zero tests; core financial logic (overspend detection, category limits)
2. account.py error paths — overdraft guard, invalid deposit/withdraw, history tracking
3. utils.py — shared helpers untested; bugs here surface everywhere
4. transaction.py validation — input guards and to_dict() serialisation

Full test cases for each area are in COVERAGE_ANALYSIS.md.

Test plan

• pip install -e ".[dev]" succeeds
• python -m pytest tests/ --cov=src/expensetracker --cov-report=term-missing -v — all 10 tests pass, coverage numbers match table above
• Adding the proposed tests from COVERAGE_ANALYSIS.md raises overall coverage above 90%

https://claude.ai/code/session_01HwTh9hP2E3nye7NBvz6p86
EOF
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