fix(lint): enable errcheck linter and fix all unchecked errors

[justin-layerv]

Summary

• Enable the errcheck linter in golangci-lint configuration
• Fix all 74 unchecked error issues across the codebase

Changes

Configuration

• .golangci.yml: Move errcheck from disable to enable list, remove test file exclusion for errcheck

nhp module (24 issues fixed)

core/scheme/curve/curve.go:

• Check SetPrivateKey error return

core/ztdo/noise.go:

• Explicitly ignore rand.Read return (crypto/rand never fails)

core/ztdo/ztdo.go:

• Handle file write errors in Encrypt and Decrypt
• Check SetMetadata and SetCipherText returns
• Explicitly ignore marshal error in toBuffer

log/logger.go:

• Explicitly ignore Output errors (logging failures are non-critical)
• Explicitly ignore file.Sync error

utils/file.go:

• Check watcher.Add error

endpoints module (50 issues fixed)

ac/:

• Handle config update function errors in watch callbacks
• Handle httpServer.Shutdown, ipset.Add, SendPacket errors
• Wrap HandleUdpACOperations goroutine call to handle error

agent/:

• Handle config update function errors in watch callbacks
• Handle preAccessRequest, processPreAccessAction errors
• Handle SendPacket, SaveStructAsJsonFile, ExitKnockRequest errors

db/:

• Handle config update function errors in watch callbacks
• Handle SetMetadata, SetNhpServer, dataPrkStore.Save errors
• Handle SendPacket, fromJson, file.Write errors
• Wrap HandleUdpDataKeyWrappingOperations goroutine call to handle error

server/:

• Handle config update function errors in watch callbacks
• Handle all updateXxx function errors in etcd config update

Test plan

• golangci-lint run ./... passes on both modules with 0 issues
• All GitHub Actions checks pass

Closes #1372

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