[tests] extism Eval() error paths need expanded mocking for plugin/instance failures

[robbyt]

Summary

engines/extism/evaluator/evaluator.go:133-134 carries a stale TODO:

// Eval implements evaluation.Evaluator
// TODO: Some error paths in this method are hard to test with the current design
// Consider adding more integration tests to cover these paths.

The TODO is being removed under #99, but the underlying gap deserves a real issue. Coverage for the package is ~93%; the remaining branches in Eval() and the helpers it calls (exec, execHelper) aren't easily reachable today because the path to a real failing WASM instance runs through a concrete *extismSDK.CompiledPlugin.

Branches needing better coverage

Eval() (already largely testable)

Most early guards are tested:

• execUnit == nil (L137-138) ✓
• GetContent() == nil (L141-143) ✓
• GetByteCode() == nil (L146-149) ✓
• exeID == "" (L152-155) — testable via custom executable unit
• type assertion failure (L159-165) — testable with a non-*compiler.Executable content
• GetExtismByteCode() == nil (L166-169) — testable
• loadInputData failure (L172-175) — testable with a mock data provider
• ConvertToExtismFormat failure (L178-181) — testable with adversarial input (e.g. unmarshalable types)
• exec(...) failure (L184-191) — partially testable via the mocked plugin pattern below

exec() and execHelper() (the harder gaps)

engines/extism/evaluator/evaluator.go:105-130 and :62-101:

File:line	Branch	Why it's hard today
evaluator.go:114-117	plugin.Instance(ctx, ...) failing	plugin is adapters.CompiledPlugin already, but tests don't yet drop in a fake whose Instance() returns an error.
evaluator.go:118-122	instance.Close(ctx) failing	Logged-only path; coverage value low.
evaluator.go:79-82	exit != 0	This branch's "should we return output?" question is already tracked under #94 and is out of scope here.
evaluator.go:73-78	instance.CallWithContext returning a non-cancellation error	Partially covered via mocked plugin instance; the cancellation-vs-other distinction (if ctx.Err() != nil) deserves explicit cases.
evaluator.go:88-91	json.Decoder.Decode failing → fallback to raw string	Trivially exercisable with non-JSON output bytes; just missing a test.

Proposal

The adapters.CompiledPlugin and adapters.SdkPluginInstanceConfig interfaces already exist in engines/extism/adapters/interfaces.go, so the seam is mostly there. Two missing pieces:

1. A reusable mock CompiledPlugin (and corresponding Instance) in the evaluator test suite, scriptable for: Instance returning an error, Instance returning a plugin whose CallWithContext errors with various shapes, Close returning an error.
2. Tests that wire a *script.ExecutableUnit whose Content is a *compiler.Executable populated with the mock plugin, then call Eval(ctx) end-to-end with each scripted failure.

Concrete subtests to add:

• TestEval_PluginInstanceError
• TestEval_CallWithContextError (separate cancelled-vs-not subcase)
• TestEval_NonJSONOutputFallsBackToString
• TestEval_ConvertToExtismFormatError (adversarial input)
• TestEval_LoadInputDataError

Out of scope

• Real production behavior — coverage / testability only.
• The exit-code-vs-output design question ([bug] Extism drops WASM output on non-zero exit code #94).
• Production WASM end-to-end via wasmdata.TestModule stays as-is; this just adds unit-level branch coverage.

Files

• engines/extism/evaluator/evaluator.go — function under test
• engines/extism/evaluator/evaluator_test.go — new subtests
• engines/extism/adapters/interfaces.go — confirm interface surface is sufficient
• New: engines/extism/evaluator/mocks_test.go (or extend existing scaffolding)

This was previously tracked inline as a TODO at evaluator.go:133-134. Filed in response to issue #99's request to either resolve TODOs or convert them to issues.