Fix CODEOWNERS review request attribution using comment metadata (#36348)

Fixes #36333

## Problem

When CODEOWNERS automatically assigns reviewers to a pull request, the
timeline incorrectly shows the PR author as the one who requested the
review (e.g., "PR_AUTHOR requested review from CODE_OWNER"). This is
misleading since the action was triggered automatically by CODEOWNERS
rules, not by the PR author.

## Solution

Store CODEOWNERS attribution in comment metadata instead of changing the
doer user:
- Add `SpecialDoerName` field to `CommentMetaData` struct (value:
`"CODEOWNERS"` for CODEOWNERS-triggered requests)
- Pass `isCodeOwners=true` to `AddReviewRequest` and
`AddTeamReviewRequest` functions
- Template can check this metadata to show appropriate attribution
message

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/issues/pull_test.go

@@ -130,7 +130,7 @@ func TestLoadRequestedReviewers(t *testing.T) {
 	user1, err := user_model.GetUserByID(t.Context(), 1)
 	assert.NoError(t, err)
 
-	comment, err := issues_model.AddReviewRequest(t.Context(), issue, user1, &user_model.User{})
+	comment, err := issues_model.AddReviewRequest(t.Context(), issue, user1, &user_model.User{}, false)
 	assert.NoError(t, err)
 	assert.NotNil(t, comment)