Tighten wazero guard lifecycle semantics and document start suppression

[Copilot]

This PR addresses the wazero review follow-ups in the guard runtime: it makes guard failure state observable, aligns test/runtime configuration, and hardens cleanup behavior during cancellation. It also documents one non-obvious instantiation choice that is easy to regress during future refactors.

• Guard lifecycle

  • add (*WasmGuard).IsHealthy() bool to expose whether a prior WASM trap has poisoned the instance
  • preserve cleanup on shutdown by closing the module/runtime with context.WithoutCancel(ctx), so caller cancellation does not skip resource release

• Instantiation behavior

  • document why WithStartFunctions() is called with no args: guard load should not auto-run _start and accidentally block on stdin or perform side effects during instantiation

• Test fidelity

  • switch the context-cancellation test to wazero.NewRuntimeConfigCompiler() so it matches production runtime construction
  • add focused coverage for IsHealthy() and for closing a guard after the caller context has already been cancelled

moduleConfig := wazero.NewModuleConfig().
    WithName(name).
    // WithStartFunctions with no args suppresses automatic _start execution
    // so guard loading cannot block on stdin or perform unexpected I/O.
    WithStartFunctions()

func (g *WasmGuard) IsHealthy() bool {
    g.mu.Lock()
    defer g.mu.Unlock()
    return !g.failed
}

[Copilot]

Pull request overview

This PR tightens WASM guard lifecycle behavior in internal/guard by preventing unintended _start execution during instantiation, exposing guard “poisoned” state after traps, and ensuring cleanup proceeds even when callers cancel contexts.

Changes:

• Suppress automatic WASM _start execution during guard instantiation via WithStartFunctions() and document the rationale.
• Add (*WasmGuard).IsHealthy() bool to expose whether a prior WASM trap has permanently failed the guard.
• Improve shutdown/cancellation fidelity by closing module/runtime with context.WithoutCancel(ctx) and aligning a cancellation test with the compiler runtime config.

Show a summary per file

File	Description
internal/guard/wasm.go	Documents _start suppression, adds IsHealthy(), and hardens cleanup to ignore caller cancellation during Close.
internal/guard/wasm_test.go	Updates runtime config in cancellation test and adds coverage for IsHealthy() plus Close behavior under canceled contexts.

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• Files reviewed: 2/2 changed files
• Comments generated: 0