Fixes Windows MCP Setup failures when installing tunnel-client.

[xingxingshese]

Summary

Fixes Windows MCP Setup failures when installing tunnel-client.

• Add bounded retries for Windows EBUSY/EPERM cleanup failures.
• Ensure staged tunnel-client files are closed before install and cleanup.
• Preserve the original setup error when rollback cleanup also fails.
• Reuse an existing valid tunnel-client.exe when possible.
• Clean up stale .install-*.exe files safely.
• Add regression tests for retry, rollback, cleanup, and existing binary reuse.

Root Cause

Windows may briefly keep a lock on the downloaded executable after it is executed for version verification. The immediate cleanup then fails with EBUSY. Previously, the rollback cleanup error could replace the original setup failure.

Verification

• bun run verify
• Windows package built successfully with electron-builder.
• Verified tunnel-client.exe --version.
• Verified no leftover tunnel-client.exe.install-*.exe files remain.