Dashboard date filters accept invalid dates

### Feature Summary

Dashboard date filters use lenient parsing and do not translate parse failures into client errors.

Before: a non-date value returns 500, while an impossible calendar date is silently normalized and returns 200.
After: malformed and impossible dates return 400, while valid ISO dates remain accepted.

Reproduction evidence:

1. Start the local development backend from main.
2. Request `GET /api/dashboard/publicSearch?resourceType=workflow&createDateStart=bogus` and observe HTTP 500.
3. Request the endpoint with `modifiedDateEnd=2026-99-99` and observe HTTP 200.
4. Request the endpoint with `createDateStart=2026-01-01` and observe HTTP 200.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT (main)

**Commit Hash (Optional)**

98588bf2ab49aac65ebf002496eb057670e6fc07

**What browsers are you seeing the problem on?**

Not browser-specific.

**Relevant log output**

The shared date parser is lenient and exposes parsing failures as internal errors.

### Proposed Solution or Design

After: malformed and impossible dates return 400, while valid ISO dates remain accepted.

### Affected Area

Workflow Engine (Amber)