Add Go fuzz tests for OSS-Fuzz integration

[canolgun]

Adds Go native fuzz targets for OSS-Fuzz integration (google/oss-fuzz#15678).

Criticality: golang.org/x/net provides DNS, HTTP/2 HPACK, and WebSocket implementations. Every Go HTTP/2 server depends on these packages. A parsing bug = pre-auth DoS vector.

Fuzz targets:

• FuzzDNSMessageParse — DNS message parser with arbitrary bytes (pre-auth boundary)
• FuzzHpackDecode — HTTP/2 HPACK header decoder (every HTTP/2 connection)
• FuzzHpackRoundTrip — HPACK encode→decode consistency

Verified: go test -fuzz=. -fuzztime=30s

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[gopherbot]

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Patch Set 1:

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