feat(api): add q parameter to list branches API for server-side filtering (#37982)

The GET /repos/{owner}/{repo}/branches endpoint currently has no way to
filter branches by name server-side, forcing API consumers to paginate
through all branches and filter client-side.

The UI already supports branch search (added in
[#27055](https://github.com/go-gitea/gitea/pull/27055)). The underlying
DB layer has a Keyword field on FindBranchOptions in
models/git/branch_list.go that does a LIKE %keyword% SQL filter, it just
wasn't wired up to the API handler.

This PR exposes a ?q= query parameter on the endpoint that maps to
FindBranchOptions.Keyword.

Example:

```GET /repos/owner/repo/branches?q=feature ```
Closes #37981

---------

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

tests/integration/integration_test.go

@@ -323,6 +323,10 @@ func logUnexpectedResponse(t testing.TB, recorder *httptest.ResponseRecorder) {
 	}
 }
 
+// DecodeJSON decodes then response as JSON into typed variable and return it
+// HINT: don't use it on existing variable (reuse existing variable):
+// if the existing var already contains some values but the new input doesn't, then it leads to wrong test result in edge cases.
+// For slice decoding, use: v := DecodeJSON(t, resp, []T{})
 func DecodeJSON[T any](t testing.TB, resp *httptest.ResponseRecorder, v T) (ret T) {
 	t.Helper()