test: validate milestone completion timestamp (#7017)

[atomantic]

Summary

• Strengthen milestone completion coverage so a completed milestone must include a non-null timestamp.
• Verify the returned value is a string containing a parseable timestamp.

Tests

• npm test --prefix server -- services/identity.test.js -t "should complete a milestone"
• npm test --prefix server -- services/identity.test.js
• Mutation probe: disabling the production timestamp assignment failed the test as expected.

Closes #7017