Fix DNS rewrite CNAMEs for DHCP hostnames

[puneetdixit200]

Closes #8068.

This updates the unresolved DHCP-host path to re-check the current request name after filtering rewrites a local-domain query to a CNAME target. When the rewritten name is a known DHCP hostname, the server now returns the local DHCP answer and lets post-filtering restore the original question and prepend the CNAME.

Tests:

• go test ./internal/dnsforward -run TestServer_ProcessDHCPHosts_rewrittenCNAME -count=1
• go test ./internal/dnsforward ./internal/filtering -count=1
• go test -race ./internal/dnsforward -run 'TestServer_ProcessDHCPHosts|TestServer_ProcessUpstream' -count=1\n- make go-lint\n- git diff --check\n\nNotes:\n- make go-lint prints existing Go 1.26.3 standard-library govulncheck findings (GO-2026-5039, GO-2026-5037), and the target still passes with its configured non-reproducible vulnerability-scan ignore.\n- make go-test reached the touched internal/dnsforward package successfully, then failed later in internal/home because TestWeb_HandleGetProfile/add_user timed out after 1m30s while running bcrypt under the race detector. The isolated retry go test -race ./internal/home -run TestWeb_HandleGetProfile -count=1 -timeout 5m passed.