test: fix SSRF expansion mock injection and error assertions

[jithinraj]

Summary

Corrects SSRF expansion test wiring and assertions in @peac/net-node.

This PR:

• fixes mock dependency injection in SSRF expansion tests so mocked DNS/HTTP paths are actually used
• updates the IPv6 private-range assertion to verify the specific SSRF error code preserved by the call chain
• corrects the empty-hostname test case so it uses a truly invalid URL

No runtime behavior, protocol surface, or public API contract is changed by this PR.

Changes

• replace dnsResolver / httpClient test options with the correct internal injection fields:
  • _dnsResolver
  • _httpClient
• update the IPv6 private-range assertion to expect E_SSRF_DNS_RESOLVED_PRIVATE
• replace https:///path with https:// in the empty-hostname test

Validation

• vitest run packages/net/node/tests/ssrf-expansion.test.ts passes
• pnpm --filter @peac/net-node exec vitest run passes
• full repo checks pass:
  • lint
  • build
  • test
  • guard

Closes #483