The DateAdapter was using a DateTimeFormatter with a fixed [.SSS] pattern,
which only supports optional 3-digit milliseconds. This caused dates with
6-digit microsecond precision (e.g., "2025-09-23T15:48:37.235982Z") sent
by the backend to silently fail parsing (returning null).

Changed to use Instant.parse() which correctly handles all valid ISO 8601
date formats including:
- No fractional seconds: 2020-01-01T00:00:00Z
- 1-9 digit fractional seconds (beyond ms precision truncated by Date)
- UTC offsets: 2023-01-01T01:00:00+01:00

Added comprehensive unit tests for various date precisions and updated
smoke test data with microsecond-date-test flag and test cases.