refactor: extract helper functions from SearchIssues (#36158)

## Summary

This PR refactors the `SearchIssues` function in
`routers/api/v1/repo/issue.go` by extracting common logic into reusable
helper functions:

- `parseIssueIsClosed()`: Parses the "state" query parameter and returns
the corresponding `isClosed` option
- `parseIssueIsPull()`: Parses the "type" query parameter and returns
the corresponding `isPull` option
- `buildSearchIssuesRepoIDs()`: Builds the list of repository IDs for
issue search based on query parameters

### Benefits:
- Improved code readability
- Smaller, more focused functions
- Easier to test individual components
- Potential for reuse in other handlers

### Changes:
- Extracted 3 helper functions from the ~292 line `SearchIssues`
function
- No functional changes - behavior remains the same
- Proper error handling preserved

## Test plan
- [ ] Verify existing API tests pass
- [ ] Manual testing of `/repos/issues/search` endpoint

Ref: #35015

---------

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

routers/web/repo/issue_list.go

@@ -25,6 +25,7 @@ import (
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/routers/common"
 	"code.gitea.io/gitea/routers/web/shared/issue"
 	shared_user "code.gitea.io/gitea/routers/web/shared/user"
 	"code.gitea.io/gitea/services/context"
@@ -45,15 +46,7 @@ func SearchIssues(ctx *context.Context) {
 		return
 	}
 
-	var isClosed optional.Option[bool]
-	switch ctx.FormString("state") {
-	case "closed":
-		isClosed = optional.Some(true)
-	case "all":
-		isClosed = optional.None[bool]()
-	default:
-		isClosed = optional.Some(false)
-	}
+	isClosed := common.ParseIssueFilterStateIsClosed(ctx.FormString("state"))
 
 	var (
 		repoIDs   []int64
@@ -268,15 +261,7 @@ func SearchRepoIssuesJSON(ctx *context.Context) {
 		return
 	}
 
-	var isClosed optional.Option[bool]
-	switch ctx.FormString("state") {
-	case "closed":
-		isClosed = optional.Some(true)
-	case "all":
-		isClosed = optional.None[bool]()
-	default:
-		isClosed = optional.Some(false)
-	}
+	isClosed := common.ParseIssueFilterStateIsClosed(ctx.FormString("state"))
 
 	keyword := ctx.FormTrim("q")
 	if strings.IndexByte(keyword, 0) >= 0 {
@@ -580,17 +565,10 @@ func prepareIssueFilterAndList(ctx *context.Context, milestoneID, projectID int6
 		}
 	}
 
-	var isShowClosed optional.Option[bool]
-	switch ctx.FormString("state") {
-	case "closed":
-		isShowClosed = optional.Some(true)
-	case "all":
-		isShowClosed = optional.None[bool]()
-	default:
-		isShowClosed = optional.Some(false)
-	}
+	isShowClosed := common.ParseIssueFilterStateIsClosed(ctx.FormString("state"))
+
 	// if there are closed issues and no open issues, default to showing all issues
-	if len(ctx.FormString("state")) == 0 && issueStats.OpenCount == 0 && issueStats.ClosedCount != 0 {
+	if ctx.FormString("state") == "" && issueStats.OpenCount == 0 && issueStats.ClosedCount != 0 {
 		isShowClosed = optional.None[bool]()
 	}