fix: reject invalid calendar date strings

[Aias00]

Purpose of the pull request

Closed: #1157

Default date conversion silently normalized invalid calendar strings instead of reporting malformed input.

What's changed?

• Disable lenient parsing for cached SimpleDateFormat instances.
• Build java.time formatters with strict resolver semantics and a default CE era so existing yyyy patterns remain valid on Java 8.
• Add DateUtils and converter tests for invalid month-end/leap-day values and valid controls.

Verification:

DateUtilsTest, DateStringConverterTest, LocalDateStringConverterTest
Tests run: 88, Failures: 0, Errors: 0, Skipped: 0
spotless:check: passed

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.