Fix eager loading: name the file after its constant

The eager-load gate caught this on its own author: the file was
field_format.rb while the class is DatetimeFormat, so Zeitwerk expected
RedmineIssueDatetime::FieldFormat and a production boot would have died
with a NameError. Every test passed, because test and development load
lazily - exactly the gap the gate exists for, and the same shape as
karida-org/redmine_gtt_sync#81.

Renamed to datetime_format.rb, with a comment so the next person does not
reintroduce it. Also restores the previous User.current in the test
teardown rather than assuming it was nil.

Author: dkastl

lib/redmine_issue_datetime.rb

@@ -1,5 +1,5 @@
 require_relative 'redmine_issue_datetime/drift_check'
-require_relative 'redmine_issue_datetime/field_format'
+require_relative 'redmine_issue_datetime/datetime_format'
 require_relative 'redmine_issue_datetime/issue_extension'
 require_relative 'redmine_issue_datetime/issue_query_extension'
 require_relative 'redmine_issue_datetime/hooks'

…b/redmine_issue_datetime/field_format.rb …edmine_issue_datetime/datetime_format.rblib/redmine_issue_datetime/field_format.rb renamed to lib/redmine_issue_datetime/datetime_format.rb lib/redmine_issue_datetime/field_format.rb renamed to lib/redmine_issue_datetime/datetime_format.rb

@@ -12,6 +12,10 @@ module RedmineIssueDatetime
   # it keeps ISO strings sortable as plain strings so ordering needs no special
   # casing, and it is consistent with this plugin showing one clock for everyone
   # rather than converting per viewer.
+  # Named DatetimeFormat in datetime_format.rb on purpose: Redmine adds every
+  # plugin's lib/ as an eager-load path, so Zeitwerk derives the constant from
+  # the filename. A mismatch here passes every test (test and development load
+  # lazily) and then kills a production boot with a NameError.
   class DatetimeFormat < Redmine::FieldFormat::Unbounded
     add 'datetime'
     self.form_partial = 'custom_fields/formats/datetime'

test/unit/datetime_format_test.rb

@@ -85,15 +85,18 @@ def setup
   # Values are shown on one clock for everyone, so formatting must not follow the
   # viewer's own time zone the way Redmine's format_time helper would.
   test 'formatting does not follow the viewer time zone' do
+    # Captured before anything that can raise, so the ensure block always
+    # restores rather than assuming what the surrounding state was.
+    previous_user = User.current
     user = User.find(2)
-    original = user.pref.time_zone
+    original_zone = user.pref.time_zone
     User.current = user
     user.pref.update(time_zone: 'UTC')
 
     assert_includes @field.format.formatted_value(nil, @field, '2026-08-03T09:15'), '09:15'
   ensure
-    user&.pref&.update(time_zone: original)
-    User.current = nil
+    user&.pref&.update(time_zone: original_zone)
+    User.current = previous_user
   end
 
   test 'formatting a blank value yields an empty string, not an error' do