[tests] extism Compile() error paths need adapter-level mocking for full coverage

[robbyt]

Summary

engines/extism/compiler/compiler.go:54-56 carries a stale TODO:

// Compile implements script.Compiler
// TODO: Some error paths are difficult to test with the current design
// Consider adding integration tests for hard-to-reach error cases.

The TODO is being removed under #99, but the underlying gap deserves a real issue. Coverage for the package is currently ~93%; the remaining branches in Compile() aren't testable today because real Extism SDK objects are constructed inside compile.CompileBytes() and exposed only through concrete types.

Specific untested branches in Compile()

File:line	Branch	Why it's hard to test today
compiler.go:63-66	io.ReadAll(scriptReader) returning a non-EOF error	Easy fix — a custom io.ReadCloser whose Read() returns an error will exercise it.
compiler.go:68-71	scriptReader.Close() returning an error	Same approach as above.
compiler.go:91-94	plugin.Instance(c.ctx, PluginInstanceConfig{}) failing	Hard. plugin is a real *extismSDK.CompiledPlugin returned by compile.CompileBytes; there's no adapter boundary to substitute a failing instance constructor.
compiler.go:96-98	instance.Close(ctx) returning an error	Logged-only, doesn't propagate, so coverage value is low.
compiler.go:103-108	instance.FunctionExists(funcName) returning false	Partially covered by TestCompile_MissingFunction but only via a real WASM module that lacks the function — the negative case isn't exercised at the unit level.
compiler.go:113-115	NewExecutable(...) returning nil	Currently unreachable: the constructor never returns nil. Either the guard is dead code or NewExecutable is intended to grow nil-returning paths.

Proposal

Extend the existing engines/extism/adapters interface set so compile.CompileBytes returns an adapters.CompiledPlugin rather than a concrete *extismSDK.CompiledPlugin. The Compiler would then accept a plugin-factory option (defaulting to the real SDK call) so unit tests could inject a mock plugin whose Instance() errors, whose FunctionExists() returns false, etc.

Concretely:

1. Make compile.CompileBytes return adapters.CompiledPlugin (the interface already exists in engines/extism/adapters/interfaces.go).
2. Add an internal seam on Compiler (e.g. compileFn func(ctx, []byte, *Settings) (adapters.CompiledPlugin, error)) so tests can override.
3. Add a mock CompiledPlugin in engines/extism/compiler/mocks_test.go and write the missing branch tests.

Out of scope

• Production behavior — this is a coverage / testability change.
• The exit-code-vs-output question is tracked separately under [bug] Extism drops WASM output on non-zero exit code #94.
• Doesn't replace the legitimate end-to-end wasmdata.TestModule-based tests; this just pulls the unit-level branches under test.

Files

• engines/extism/compiler/compiler.go — function under test
• engines/extism/compiler/internal/compile/compile.go — return type adjustment
• engines/extism/adapters/interfaces.go — confirm CompiledPlugin covers the surface needed
• engines/extism/compiler/compiler_test.go — new branch tests
• New: engines/extism/compiler/mocks_test.go (or extend existing test scaffolding)

This was previously tracked inline as a TODO at compiler.go:54-56. Filed in response to issue #99's request to either resolve TODOs or convert them to issues.