Fix Windows test failures by removing .cmd fixtures and adding test bridge injection

[mmprotest]

Motivation

• Tests on Windows were failing with spawn EINVAL because test fixtures generated .cmd wrappers or wrote batch text to files named *.exe, which Node cannot spawn with shell: false and are not real executables.
• The goal is to make the test harness cross-platform without changing production runtime launching semantics (spawn(..., { shell: false })).

Description

• Replace generated .cmd/shell-wrapper test fixtures with temporary Node bridge modules and CommandSpec usage in integrations/pi-villani/src/process.test.ts so tests invoke process.execPath directly; updated file: process.test.ts.
• Add a scoped test-only bridge starter injection hook to integrations/pi-villani/src/index.ts (bridgeStarter + __setBridgeStarterForTests) so tests can inject a mock starter while production defaults to startVillaniBridgeProcess.
• Update integrations/pi-villani/src/extension.test.ts to install a mock bridge starter that launches the temporary bridge.mjs via process.execPath, to avoid creating .cmd wrappers; ensure injected starters/prompters are restored in every finally block and add a waitForCondition helper for synchronization.
• Change the cached-runtime test to create a verified cached-runtime placeholder file that is not executed and assert the resolved cached path is passed to the bridge starter (avoid writing batch text into villani-code.exe).
• Preserve production subprocess safety: no changes to production spawn options (still uses shell: false).
• Modified files: integrations/pi-villani/src/index.ts, integrations/pi-villani/src/process.test.ts, integrations/pi-villani/src/extension.test.ts.

Testing

• Ran cd integrations/pi-villani && npm test; result: test suite passed (38 tests, 38 passed, 0 failed).
• Ran cd integrations/pi-villani && npm run build; result: TypeScript build succeeded.
• Attempted cd integrations/pi-villani && pi install . in this environment but pi is not installed here, so it could not be executed (not run).

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Codex Task