fix: walk error cause chain and add no-hyphen SSL certificate error variant

[sonukapoor]

Follow-up to #394. The SSL error hint was not showing on some corporate machines for two reasons:

1. No-hyphen variant missing — Node.js produces "self signed certificate in certificate chain" (no hyphen) but the matcher only checked "self-signed certificate" (with hyphen). Both variants are now covered.

2. Error cause chain not walked — Node.js fetch wraps the SSL error in a cause chain: the top-level error says "fetch failed" and the SSL detail is in error.cause.message. The previous implementation only read error.message, missing the cause. Added extractErrorMessage() to network.ts that recursively walks the cause chain, and updated both catch blocks in osv-advisory-source.ts to use it.

Closes #394