fix(channels example): pi adapter rejects unsupported attachments, keeps message ids across forks

[cjol]

This PR fixes two gaps in the pi harness adapter of the Channels example (piChannelsHarness in examples/next/channels/src/pi/channels-harness.ts): dropped attachments and lost message ids after a fork. It also adds the example's first test setup.

Why

• toUserInput silently dropped any part that was not an inline base64 image, so pi answered a prompt missing the PDF or remote image the user sent, with no sign anything was lost.
• After a fork, inherited user messages showed under pi entry ids instead of the caller's message ids. resolve only looked at the fork's own submissions and cache namespace, but the inherited entries were placed by the parent session.
• For the fork fix, two options: copy the entry-to-message mappings at fork time, or resolve each entry against the session that placed it. pi keeps the original conversationId on inherited entries, so resolving by that needs no fork hook, covers forks of forks, and falls back to pi's own submission records after eviction. Copying would need a scan at fork time and would miss mappings not yet cached.

Code Changes

• toUserInput now throws for parts pi cannot take (non-image, non-text files, or files by URL), naming the media type. The submit fails before anything is recorded, so Channels rejects the event.
• Inline text/* files are decoded into text parts, since pi takes text. Images use the part's mediaType and accept data URLs with extra parameters.
• transcript() groups user entries by entry.conversationId and resolves each group against that session's operation records.
• src/tests/: a Workers-pool vitest with a Durable Object running PiHarness on pi-ai's faux provider, modelled on examples/next/harnesses/pi/src/tests. Adds test and typecheck scripts, and the vitest pool types to the example's tsconfig.json.

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