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/api/v1/repo/milestone.go

@@ -10,7 +10,6 @@ import (
 
 	"code.gitea.io/gitea/models/db"
 	issues_model "code.gitea.io/gitea/models/issues"
-	"code.gitea.io/gitea/modules/optional"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/web"
@@ -60,12 +59,7 @@ func ListMilestones(ctx *context.APIContext) {
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 
-	state := api.StateType(ctx.FormString("state"))
-	var isClosed optional.Option[bool]
-	switch state {
-	case api.StateClosed, api.StateOpen:
-		isClosed = optional.Some(state == api.StateClosed)
-	}
+	isClosed := common.ParseIssueFilterStateIsClosed(ctx.FormString("state"))
 
 	milestones, total, err := db.FindAndCount[issues_model.Milestone](ctx, issues_model.FindMilestoneOptions{
 		ListOptions: utils.GetListOptions(ctx),