fix(think): preserve attachment fetchMetadata through messenger serialization

[threepointone]

Summary

Fixes #1833.

When a conversation resolver routes a thread to a sub-agent Durable Object, the messenger event is run through serializableMessengerEvent() before crossing the DO boundary. That serialization dropped everything except id, mediaType, name, size, text, and url from each attachment — discarding fetch, raw, and the platform identifier.

For adapters like @chat-adapter/telegram, the file id lives exclusively in fetchMetadata.fileId and the top-level id is never populated, so photos became irretrievable inside a sub-agent (attachment.id and attachment.fetch were both missing).

This change:

• Adds a serialization-safe fetchMetadata?: Record<string, string> field to MessengerAttachment.
• Preserves fetchMetadata through serializableMessengerEvent (the exact spot identified in the issue), while still dropping the non-cloneable fetch/raw/data.
• Carries fetchMetadata through toMessengerAttachment, and backfills the top-level id from a known, adapter-agnostic metadata key (id/fileId/mediaId/fileUniqueId).

This implements both options the issue suggested (preserve fetchMetadata and backfill id), so a downstream sub-agent has the data needed to re-fetch via the adapter's rehydrateAttachment().

Test plan

• Extended the serialization-snapshot test to assert fetchMetadata survives JSON round-tripping while raw/fetch/data are still stripped.
• Added focused toMessengerAttachment tests: id backfill from fetchMetadata.fileId, and id staying undefined when no known id key is present.
• pnpm run check (sherif + export checks + format + lint + typecheck, all 114 projects) passes.
• @cloudflare/think test suite passes (894 unit/integration + generated/vite/cli/react).
• Added a @cloudflare/think patch changeset.

Made with Cursor

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🦋 Changeset detected

Latest commit: 5995df3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@cloudflare/think	Patch

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commit: 5995df3