ci: add windows-latest to test matrix (P1 prevention)

[refined-element]

Why

This SDK's CI ran Linux-only, and that gap is what let the auth-bypass ship: the old secp256k1 dependency built fine on Linux but failed to install on Windows, and CI never noticed. The backend is now coincurve (ships prebuilt wheels for all platforms), so Windows should work — this change proves it on every PR and prevents the class of regression from recurring.

What changed

Converted the test job in both test.yml and publish.yml from a 1-D Python-version matrix on ubuntu-latest to a 2-D OS × Python matrix:

   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
+        os: [ubuntu-latest, windows-latest]
         python-version: ["3.10", "3.11", "3.12"]

• 6 legs per workflow (2 OS × 3 Python) instead of 3.
• Steps unchanged: pip install -e ".[dev]" and python -m pytest tests/ -v are both cross-platform (no bash-only shell steps in the test job).
• The publish job stays ubuntu-latest (it has bash-specific steps and is not part of the test matrix — untouched).

Local Windows verification

Ran the exact CI leg locally on Windows (Python 3.12, win32) in a clean venv:

• pip install -e ".[dev]" succeeded — coincurve installed from a prebuilt wheel, no native build toolchain needed.
• python -m pytest: 191 passed (matches the Linux baseline of 191, no failures).

🤖 Generated with Claude Code

https://claude.ai/code/session_01CYXrdWWjKo5AyGqFAfrdgK