Fix: Improve RFC3339 date parsing validation (Issue #2418)

- Replace search() with fullmatch() for strict RFC3339 format validation
- Provide clear, actionable error messages with expected format
- Add input whitespace stripping before validation
- Improve exception handling with specific ValueError messages
- Add comprehensive test cases for invalid formats
- Addresses reviewer feedback from PR #2420

Test: Existing tests pass + new test cases added