chore(fixtures): add a test Workflow and pin the Workflow binding's behaviour in @observerly/orderly

[michealroberts]

What This Changes

• Adds Echo, a test Workflow that reports the type of each param it receives, bound as ECHO with unknown params, as wrangler types writes them.
• Adds settled(), a helper that reads an instance's status until it is complete or errored.
• Adds tests/workflow-binding.spec.ts, which pins how the local engine treats instance ids, a missing id, Dates and repeated ids, and which binding types the compiler accepts.

Notes For Reviewers

• Tests and configuration only, with an empty changeset. Nothing in src changes.
• Locally, creating an instance with an id that already exists starts nothing and doesn't throw.

Checklist

• Tests cover the change, and run inside workerd
• A changeset is included, or the change is not one a consumer would notice
• No Node built-ins were introduced