feat: PATCH semantics for MergeRequest::$autoMergeStrategy

- DMD-1257
- Omitting autoMergeStrategy from the update request now preserves the
  existing value instead of resetting it to disabled.
- New enum value `none` allows explicitly disabling auto-merge.
- DB column changed to NOT NULL DEFAULT 'none'.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: martinsifra

apiary.apib

@@ -9955,6 +9955,7 @@ other buckets (production buckets or buckets belongs the other development branc
         + Members
             + `immediately`
             + `scheduled`
+            + `none`
     + autoMergeAt (optional, string) - ISO 8601 datetime for scheduled auto-merge, required when autoMergeStrategy is "scheduled" (e.g. "2026-04-01T12:00:00+00:00")
 
 + Request (application/json)
@@ -10017,10 +10018,11 @@ other buckets (production buckets or buckets belongs the other development branc
     + title (required, string) - title of the merge request
     + description (optional, string) - description of the merge request
     + externalId (optional, string) - identifier of the merge request in an external system; omit to keep unchanged, send empty string to clear
-    + autoMergeStrategy (optional, enum[string]) - auto-merge strategy
+    + autoMergeStrategy (optional, enum[string]) - auto-merge strategy; omit to keep unchanged, send "none" to disable
         + Members
             + `immediately`
             + `scheduled`
+            + `none`
     + autoMergeAt (optional, string) - ISO 8601 datetime for scheduled auto-merge, required when autoMergeStrategy is "scheduled" (e.g. "2026-04-01T12:00:00+00:00")
 
 + Request (application/json)