MODEXPW-651: Always format email context createdAt with millisecond precision

[julianladisch]

https://folio-org.atlassian.net/browse/MODEXPW-651

Purpose

OrderEmailContextMapperTest.buildContext_setsCreatedAt:177 fails in about .1 % of all runs with this error:

Expecting actual:
  "2026-08-11T15:33:47Z"
to match pattern:
  "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z"

Example: https://github.com/folio-org/mod-data-export-worker/actions/runs/31503434083/job/93819003252

Cause: Instant.toString() omits the fractional part when the nano-of-second is zero, so roughly one order email in a thousand carried a createdAt without milliseconds and OrderEmailContextMapperTest failed at the same rate.

Approach

A fixed 3-digit millisecond instant formatter makes the output deterministic.

Pre-Merge Checklist:

Before merging this PR, please go through the following list and take appropriate actions.

• Does this PR meet or exceed the expected quality standards?
  • Code coverage on new code is 80% or greater
  • Duplications on new code is 3% or less
  • Check logging
  • There are no major code smells or security issues
• Does this introduce breaking changes?
  • n/a Were any API paths or methods changed, added or removed?
  • n/a Were there any schema changes?
  • n/a Did any of the interface versions change?
  • n/a Were permissions changed, added, or removed?
  • n/a Are there new interface dependencies?
  • There are no breaking changes in this PR.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[markusweigelt]

Thx for your work!