Upgrade Vitest to v4 and improve Windows path handling

[czenzel]

This pull request includes several dependency updates, cross-platform path handling improvements, and enhancements to the spawn-workerd integration test utility to better support Windows environments. The most important changes are summarized below:

Dependency Updates:

• Updated vitest and @vitest/coverage-v8 to version 4.1.10 in multiple package.json files for improved testing features and compatibility. [1] [2]
• Updated @cloudflare/workers-types to ^5.20260801.1 in template and app packages. [1] [2]
• Updated autocannon to ^2.0.1 in groundflare package.

Cross-Platform Path Handling:

• Replaced Node's path.join with path/posix.join and normalized path separators to / in BootstrapStateStore and FileSecretStore to ensure consistent behavior on Windows and Unix systems. Added a helper normalizePortablePath for this purpose. [1] [2] [3] [4] [5]

Integration Test Improvements (Windows Support):

• Enhanced spawn-workerd.ts to select the correct native workerd binary for the current platform and architecture, including direct execution of the .exe on Windows to avoid orphaned child processes and locked temp directories. [1] [2]
• Added a robust removeWorkdir function with retry logic to handle Windows file handle delays during test cleanup. Updated all relevant cleanup calls to use this function. [1] [2] [3] [4] [5]

Test Configuration Simplification:

• Simplified vitest configuration files by removing unnecessary poolOptions nesting and clarifying comments. [1] [2]