ci(mcp-proxy): add cross-platform test matrix

Extend the existing test workflow to run the full Python 3.10-3.13 suite on Linux, Windows, and macOS runners with fail-fast disabled so platform-specific failures remain visible.

Use python -m pip and python -m pytest for shell-neutral invocation across runners. No product code or test markers were changed in this slice.

Implemented with assistance from Codex.

Author: oleg-bk

.github/workflows/tests.yml

@@ -8,9 +8,11 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
+      fail-fast: false
       matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
         python-version: ["3.10", "3.11", "3.12", "3.13"]
 
     steps:
@@ -22,7 +24,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Install dependencies
-        run: pip install -e ".[test]"
+        run: python -m pip install -e ".[test]"
 
       - name: Run tests
-        run: pytest -v
+        run: python -m pytest -v