refactor: adopt go 1.27 idioms

[amir20]

Follow-up to #4934 now that the toolchain is on go 1.27.

• errors.AsType instead of declaring a target var and calling errors.As
• promoted fields set directly in composite literals, now that 1.27 allows it (docker HostConfig resources, network.Summary, k8s TypeMeta/ObjectMeta, embedded test fixtures)
• strings.Cut instead of strings.Split when splitting env vars on =
• separate commit for go mod tidy, which had drifted on master

go build, go vet and go test ./internal/... all pass on 1.27.0.

[claude]

Reviewed. Clean, mechanical refactor, no functional changes.

• errors.AsType[*json.UnmarshalTypeError](err) in event_generator.go:320 correctly replaces the declare+errors.As pattern, single usage so errors import still needed.
• Promoted-field literals (deployer.go HostConfig, manager_test.go network.Summary, k8s client_test.go TypeMeta/ObjectMeta) all target genuinely embedded structs, gofmt alignment looks correct.
• strings.Cut(env, "=") in valid_env.go:26 is equivalent to the old strings.Split(env, "=")[0].
• Couldn't execute go build/go vet in this sandbox to double check errors.AsType compiles against your 1.27 toolchain, but PR description says it was verified locally.

No bugs, security, or test coverage concerns. LGTM.