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/common/issue_filter.go

@@ -0,0 +1,25 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"code.gitea.io/gitea/modules/optional"
+)
+
+func ParseIssueFilterStateIsClosed(state string) optional.Option[bool] {
+	switch state {
+	case "all":
+		return optional.None[bool]()
+	case "closed":
+		return optional.Some(true)
+	case "", "open":
+		return optional.Some(false)
+	default:
+		return optional.Some(false) // unknown state, undefined behavior
+	}
+}
+
+func ParseIssueFilterTypeIsPull(typ string) optional.Option[bool] {
+	return optional.FromMapLookup(map[string]bool{"pulls": true, "issues": false}, typ)
+}