refactor: extract helper functions from SearchIssues

[bimakw]

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