wit-component: fix async task.return options for export signatures

[peter-jerry-ye]

Hi maintainers,

DISCLAIMER : I generated this fix using AI

This PR addresses #2301 by ensuring async task.return options are derived from the full exported function signature, not only the immediate return type.

What this changes

• Adds a regression test first (async-task-return-param-string) with snapshots that make the expected task.return options explicit.
• Fixes option calculation for async export task.return by using the full Function context.

Why this fixes #2301

Issue #2301 reports invalid task.return signature/options in async scenarios involving strings/encodings. This change propagates required canonical ABI options from the relevant async signature data, so task.return is emitted with the correct options in these cases.

Notes

• Commit order is intentionally: regression test first, fix second.
• The new regression includes snapshot output to show the behavioral diff clearly.

Testing

• cargo test -p wit-component --test components async-task-return-param-string
• cargo test -p wit-component --test components async-streams-and-futures

Thank you for taking a look.

[alexcrichton]

Thanks!