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>

core/dnsserver/server_quic.go

@@ -363,7 +363,8 @@ func readDOQMessage(r io.Reader) ([]byte, error) {
 	// A client or server receives a STREAM FIN before receiving all the bytes
 	// for a message indicated in the 2-octet length field.
 	// See https://www.rfc-editor.org/rfc/rfc9250#section-4.3.3-2.2
-	if size != uint16(len(buf)) {
+	//nolint:gosec
+	if size != uint16(len(buf)) { // #nosec G115 -- buf length fits in uint16
 		return nil, fmt.Errorf("message size does not match 2-byte prefix")
 	}