test: DemoExecutor e2e tests with mock WAA environment (#222)

12 tests covering the full DemoExecutor pipeline:
- Keyboard-only demo: 3 steps execute in order, all Tier 1
- Mixed demo: click uses grounder, keyboard bypasses it
- Evaluation: dense score with milestones, binary without
- Telemetry: start/completed events with tier counts
- Edge cases: empty demo, missing values, unknown action types

All tests use MockEnv (no WAA server, no HTTP, no API keys).
12/12 pass in 0.05s.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abrichr