perf(iac): avoid redundant parsing in file type detection

[HairyMike]

Description

• Kubernetes, CloudFormation, and Terraform Plan JSON matchers previously called IsType recursively, fully parsing each candidate file multiple times during misconfiguration filtering.
• Each matcher now reads file content once and unmarshals once.

On repositories with many YAML/JSON files that are not IaC (e.g. OpenAPI specs), the detection phase alone can allocate multiple GB before any policy evaluation runs. This removes the worst redundant parses without changing the public API.

Before / after

Apple M3 Pro, GOEXPERIMENT=jsonv2, go test ./pkg/iac/detection/ -bench=BenchmarkIsType -benchmem -count=10

Benchmark	Before	After
IsType_Kubernetes_YAML_1x	27.1 µs/op, 371 allocs/op	12.4 µs/op, 186 allocs/op (−54%)
IsType_CloudFormation_YAML_1x	22.7 µs/op, 321 allocs/op	10.7 µs/op, 138 allocs/op (−53%)
Per-file detection during misconfiguration filtering now parses each candidate YAML once instead of 2–3 times.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[CLAassistant]

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