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

[bimakw]

Thanks @lunny for the review! Updated to use || operator to consolidate the error handling conditions.

[bimakw]

Thanks @wxiaoguang for the feedback! You're right - using string comparison for errors is fragile.

I've added a proper custom error type ErrOwnerOrgRequired with IsErrOwnerOrgRequired() helper function, following the same pattern used elsewhere in the codebase (e.g., ErrOrgNotExist, ErrTeamNotExist).