fix(ci): pin unit-test runners to ubuntu-24.04 for Python 3.9

ubuntu-latest now resolves to Ubuntu 26.04, where
actions/setup-python has no prebuilt Python 3.9 — the matrix fails
at the setup step. Pin the runner to ubuntu-24.04 (LTS).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: foleydang

.github/workflows/unit_test.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
+        os: [ ubuntu-24.04 ]
         python-version: [ '3.9', '3.10', '3.11', '3.12', '3.13' ]
 
     steps:
@@ -68,7 +68,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
+        os: [ ubuntu-24.04 ]
         python-version: [ '3.9', '3.10', '3.11', '3.12', '3.13' ]
 
     steps: