fix(ci): drop pip cache from agent-e2e setup-python

actions/setup-python's `cache: pip` hard-fails when the repo has no
requirements.txt or pyproject.toml to hash for the cache key — true for
this repo, where Python exists only to run mcp-testkit. Carried over
from the Python SDK's agent-e2e workflow, where a pyproject.toml exists.

First observed on PR #131: the job died in "Set up Python" before any
test ran, and the silently-empty guard correctly reported 0/0.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Kowser

.github/workflows/agent-e2e.yml

@@ -41,7 +41,8 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: '3.12'
-          cache: 'pip'
+          # no `cache: pip` — it requires a requirements.txt/pyproject.toml
+          # to key on, and this repo has neither (Python is mcp-testkit only)
 
       - name: Set up Java
         uses: actions/setup-java@v4