Update build infra to support new branching strategy (#7027)

rapids_config will use `RAPIDS_BRANCH` contents to determine what branch to use

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #7027

Author: robertmaynard

RAPIDS_BRANCH

@@ -0,0 +1 @@
+branch-25.10

cmake/RAPIDS.cmake

@@ -18,9 +18,9 @@
 cmake_minimum_required(VERSION 3.30.4 FATAL_ERROR)
 
 # Allow users to control which version is used
-if(NOT rapids-cmake-version OR NOT rapids-cmake-version MATCHES [[^([0-9][0-9])\.([0-9][0-9])$]])
+if(NOT rapids-cmake-branch OR NOT rapids-cmake-version)
   message(
-    FATAL_ERROR "The CMake variable rapids-cmake-version must be defined in the format MAJOR.MINOR."
+    FATAL_ERROR "The CMake variable `rapids-cmake-branch` or `rapids-cmake-version` must be defined"
   )
 endif()
 
@@ -33,7 +33,7 @@ endif()
 # Allow users to control which branch is fetched
 if(NOT rapids-cmake-branch)
   # Define a default branch if the user doesn't set one
-  set(rapids-cmake-branch "branch-${rapids-cmake-version}")
+  set(rapids-cmake-branch "release/${rapids-cmake-version}")
 endif()
 
 # Allow users to control the exact URL passed to FetchContent

cmake/rapids_config.cmake

@@ -25,5 +25,15 @@ else()
       "Could not determine RAPIDS version. Contents of VERSION file:\n${_rapids_version_formatted}")
 endif()
 
+# Use STRINGS to trim whitespace/newlines
+file(STRINGS "${CMAKE_CURRENT_LIST_DIR}/../RAPIDS_BRANCH" _rapids_branch)
+if(NOT _rapids_branch)
+  message(
+    FATAL_ERROR
+      "Could not determine branch name to use for checking out rapids-cmake. The file \"${CMAKE_CURRENT_LIST_DIR}/../RAPIDS_BRANCH\" is missing."
+  )
+endif()
+
 set(rapids-cmake-version "${RAPIDS_VERSION_MAJOR_MINOR}")
+set(rapids-cmake-branch "${_rapids_branch}")
 include("${CMAKE_CURRENT_LIST_DIR}/RAPIDS.cmake")