ci: run tests, real Lean + Mathlib smoke and image build on the lean4.35 runner

Moves the Lean project to Lean 4.35.0 / Mathlib v4.35.0 to match the runner,
caches the Mathlib project on the runner host, and builds and keeps the Docker
image only after the Python and Lean jobs pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FYqwNMyPUHeTbFWg83GQq6

lean/lakefile.toml

@@ -6,7 +6,7 @@ defaultTargets = ["LeanBench"]
 [[require]]
 name = "mathlib"
 git = "https://github.com/leanprover-community/mathlib4.git"
-rev = "v4.33.1"
+rev = "v4.35.0"
 
 [[lean_lib]]
 name = "LeanBench"