feat: Add CD001-MCP TaxCalc Server Tests (CD001, #27)

[steadhac]

Description

Add a full unit test suite for the TaxCalc MCP server — a stateless mock tax calculator.
Pure computation, no DB writes. Tests cover tax math correctness for all 7 jurisdictions,
category handling (goods, services, entertainment), tax rate lookups, TIN/EIN format
validation, server config overrides, and string/float edge cases.

Bug-exposing tests document missing input validation for negative amounts, unknown
categories, an unreachable SSN dead-code branch, and whitespace jurisdiction handling.

Tests follow the established pattern with:

• Title / Basically question / Steps / Expected Results
• Bug-exposing tests included for each confirmed production defect.

📁 Test Files
tests/unit/mcp/test_taxcalc.py

TestCalculateTax

Test ID	Title
test_tc_calc_001	calculate_tax returns correct tax breakdown for US-CA goods
test_tc_calc_002	Services category is tax-exempt when service_tax_exempt=True
test_tc_calc_003	Entertainment category adds the entertainment surcharge
test_tc_calc_004	Unknown jurisdiction returns error with available jurisdictions
test_tc_calc_005	Empty jurisdiction falls back to default_jurisdiction
test_tc_calc_006	US-NY calculation includes state, county, and city taxes
test_tc_calc_007	amount=0 returns zero tax for all components
test_tc_calc_008 ⚠️	Negative amount accepted without validation — produces negative tax
test_tc_calc_009 ⚠️	Unknown category accepted without validation — treated silently as goods
test_tc_calc_010	Fractional amounts rounded to 2 decimal places
test_tc_calc_011	All 7 jurisdictions compute without error
test_tc_calc_012	services not exempt when service_tax_exempt=False in config

TestGetTaxRates

Test ID	Title
test_tc_rates_001	get_tax_rates with no jurisdiction returns all 7 rates
test_tc_rates_002	Specific jurisdiction returns single entry
test_tc_rates_003	Unknown jurisdiction returns error
test_tc_rates_004	combined_rate equals sum of state+county+city
test_tc_rates_005	Response includes service_tax_exempt and entertainment_surcharge_pct
test_tc_rates_006	Custom tax_rates in server_config replace default rates

TestValidateTaxId

Test ID	Title
test_tc_tin_001	Valid 9-digit EIN returns format_valid=True, id_type=EIN
test_tc_tin_002	EIN with dashes normalized correctly
test_tc_tin_003	Tax ID not 9 digits returns format_valid=False
test_tc_tin_004 ⚠️	SSN validation branch is unreachable dead code
test_tc_tin_005	Unsupported country returns not-supported error
test_tc_tin_006	Empty tax_id returns format_valid=False
test_tc_tin_007	Spaces in tax_id stripped before validation
test_tc_tin_008	Letters in tax_id return format_valid=False

TestTaxCalcServerConfig

Test ID	Title
test_tc_cfg_001	DEFAULT_CONFIG contains all expected configuration keys
test_tc_cfg_002	DEFAULT_CONFIG.tax_rates contains exactly 7 jurisdictions
test_tc_cfg_003	Custom entertainment_surcharge_pct applied in calculations
test_tc_cfg_004	Custom default_jurisdiction used when jurisdiction is empty

TestTaxCalcToolDiscovery

Test ID	Title
test_tc_tools_001	TaxCalc server exposes exactly 3 tools
test_tc_tools_002	calculate_tax schema has amount parameter
test_tc_tools_003	validate_tax_id schema has tax_id and country

TestFloatEdgeCases

Test ID	Title
test_tc_float_001	Very large amounts (1 billion) computed without overflow
test_tc_float_002	Very small amounts (0.01) return rounded totals

TestStrEdgeCases

Test ID	Title
test_tc_str_001	SQL injection in jurisdiction handled safely
test_tc_str_002 ⚠️	Whitespace-only jurisdiction not treated as empty — missing strip
test_tc_str_003	Very long tax_id (1000 chars) handled without crash
test_tc_str_004	Unicode in tax_id returns format_valid=False without crash

Related Bug Tickets

Bug_183, Bug_184, Bug_185, Bug_186