plugin: fix gosec G115 integer overflow warnings (#7799)

Fix integer overflow conversion warnings (G115) by adding appropriate
suppressions where values are provably bounded.

Fixes: https://github.com/coredns/coredns/issues/7793

Changes:
- Updated 56 G115 annotations to use consistent // #nosec G115 format
- Added 2 //nolint:gosec suppressions for conditional expressions
- Removed G115 exclusion from golangci.yml (now explicitly handled per-line)

Suppressions justify why each conversion is safe (e.g., port numbers
are bounded 1-65535, DNS TTL limits, pool lengths, etc.)

Signed-off-by: Azeez Syed <syedazeez337@gmail.com>

plugin/test/helpers.go

@@ -117,7 +117,7 @@ func OPT(bufsize int, do bool) *dns.OPT {
 	o.Hdr.Name = "."
 	o.Hdr.Rrtype = dns.TypeOPT
 	o.SetVersion(0)
-	o.SetUDPSize(uint16(bufsize))
+	o.SetUDPSize(uint16(bufsize)) // #nosec G115 -- buffer size fits in uint16
 	if do {
 		o.SetDo()
 	}