Expand test coverage with edge cases, invalid inputs, and all configuration options

[teesofttech]

Description

The current test suite (CronCraft.Test/CronHelperTest.cs) covers only happy-path scenarios with default settings. Many important edge cases and configuration combinations are untested, which could hide bugs.

Missing Test Cases

Input Validation

• Empty string input ("")
• Null input (null)
• Too few parts ("* * *" — 3 parts)
• Too many parts ("* * * * * * * *" — 8 parts)
• Wildcard-only expression ("* * * * *")
• Invalid field values (e.g., "*/61 * * * *" — 61 minutes is invalid)
• Invalid day value (e.g., "0 0 * * 8" — day 8 doesn't exist)

Day Name Formats

• DayNameFormat = "full" — e.g., "Every Monday"
• DayNameFormat = "short" — e.g., "Every Mon"
• DayNameFormat = "single" — e.g., "Every M"
• DayNameFormat = "custom" with a custom day name map

Localization

• Spanish (es) output for at least 3 different cron patterns
• French (fr) output for at least 3 different cron patterns
• Unsupported/unknown language code falls back gracefully

Time Zone

• Valid timezone conversion (UTC → specific timezone)
• Invalid timezone string behavior

Cron Patterns

• Specific month (e.g., "0 9 1 3 *") — verifies month is included in output
• Both day-of-month and day-of-week set (e.g., "0 23 15 * 1")
• Quartz 6-part expressions
• Quartz 7-part expressions
• Step values in minute and hour fields

Where to Look

• CronCraft.Test/CronHelperTest.cs — add new test methods here

Acceptance Criteria

• All listed test cases are implemented and passing
• Each test has a clear name describing the scenario being tested
• Tests use [ExpectedException] or Assert.ThrowsException for error cases
• Test file remains organized