Add Python 3.15 and 3.15t to Linux nightly wheel builds

Add 3.15 and 3.15t (free-threaded) to the binary build matrix for Linux
wheels on the nightly channel. 3.15 is brand new and only the Linux
manywheel builders are ready for it, so:

- The versions are added to the "nightly" entry of PYTHON_ARCHES_DICT
  only; "test" and "release" stay at 3.10-3.14t until 3.15 is releasable.
- generate_wheels_matrix skips any 3.15* version when the OS is not
  linux or linux-aarch64, mirroring the per-OS exclusion previously used
  for 3.14 on Windows. macOS, Windows, and Windows arm64 are unaffected.

The conda environment setup in setup-binary-builds installs 3.15/3.15t
from the conda-forge python_dev label (where the 3.15 alphas and the
python-freethreading metapackage are published), since 3.15 is not yet on
the default Anaconda channel. This mirrors how 3.14 was first brought up
via conda-forge/label/python_rc before being switched to the default
channel once it shipped. The pinned alpha (3.15.0a8) should be bumped as
newer pre-releases are published.

The reference matrices under tools/tests/assets were regenerated for the
four Linux wheel configurations; macOS/Windows references are unchanged.

Test Plan:

```
cd tools/tests
python3 -m tools.tests.test_generate_binary_build_matrix
```

All 7 tests pass. Also verified per-OS/per-channel scoping:

```
python3 -c "from tools.scripts.generate_binary_build_matrix import generate_build_matrix; ..."
# linux/linux-aarch64 nightly include 3.15/3.15t; macos/windows do not;
# linux test/release do not.
```

This PR was authored with the assistance of an AI assistant.

Author: atalman

.github/actions/setup-binary-builds/action.yml

@@ -133,6 +133,16 @@ runs:
 
           # shellcheck disable=SC2153
           case $PYTHON_VERSION in
+            3.15t)
+              # 3.15 is pre-release; conda-forge ships it under the python_dev
+              # label. Bump the pinned alpha/beta as newer ones are published.
+              export PYTHON_VERSION=3.15.0a8
+              export CONDA_EXTRA_PARAM=" python-freethreading -c conda-forge/label/python_dev -c conda-forge"
+              ;;
+            3.15)
+              export PYTHON_VERSION=3.15.0a8
+              export CONDA_EXTRA_PARAM=" -c conda-forge/label/python_dev -c conda-forge"
+              ;;
             3.14t)
               export PYTHON_VERSION=3.14
               export CONDA_EXTRA_PARAM=" python-freethreading"

tools/scripts/generate_binary_build_matrix.py

@@ -22,7 +22,7 @@
 
 
 PYTHON_ARCHES_DICT = {
-    "nightly": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"],
+    "nightly": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t", "3.15", "3.15t"],
     "test": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"],
     "release": ["3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"],
 }
@@ -463,6 +463,10 @@ def generate_wheels_matrix(
     ret: List[Dict[str, Any]] = []
     for python_version in python_versions:
         for arch_version in arches:
+            # Python 3.15 and 3.15t are currently only built for Linux wheels.
+            if os not in (LINUX, LINUX_AARCH64) and python_version.startswith("3.15"):
+                continue
+
             gpu_arch_type = arch_type(arch_version)
             gpu_arch_version = (
                 "" if arch_version in [CPU, CPU_AARCH64, XPU] else arch_version