Standardize nil-logger handling and resolve outstanding TODOs

[robbyt]

Summary

Three helpers each handle nil-logger differently:

• platform/data/addDataToContext.go:28-31 — falls back to slog.Default(), carries a live // TODO: remove or use logger more effectively.
• platform/data/loadInputData.go:16-18 — falls back to slog.Default() silently.
• internal/helpers/logger.go (SetupLogger) — creates a default text handler over stdout/stderr and logs a warning that the handler was nil.

A library should not write to stderr unless the consumer asks for it. Pick one strategy ("nil = use a discard handler") and apply it everywhere.

Also clean up

While we're in the area, two more TODOs in non-test code:

• engines/extism/compiler/compiler.go:54 — // TODO: Some error paths are difficult to test with the current design
• engines/extism/evaluator/evaluator.go:133 — // TODO: Some error paths in this method are hard to test with the current design

Resolve them by either making the code testable (probably mocking the Extism SDK plugin / instance) or by removing the TODOs once we're satisfied with coverage. The output-on-non-zero-exit TODO at engines/extism/evaluator/evaluator.go:80 is tracked separately in #94.

Files

• internal/helpers/logger.go
• platform/data/addDataToContext.go
• platform/data/loadInputData.go
• engines/extism/compiler/compiler.go
• engines/extism/evaluator/evaluator.go