Read history on a thread that has never run as empty

[stephschofield]

Fixes #72

What

Reading message history on a thread that has never been run returns 500 {"error":"Failed to fetch thread messages"}.

A thread id is minted before the thread exists — the platform creates it on the first run — so this is the normal opening move for every new conversation, not an error. The runtime reports the platform's THREAD_NOT_FOUND as a bare 500, which reads as a broken server and buries a stack trace in the log each time somebody opens a new chat.

Reproduced against a live Intelligence deployment:

thread	state	before
d3ff669d…	has a prior run	200, history returned
55569917…	just minted, never run	500 ← platform THREAD_NOT_FOUND

How

After a GET on /threads/:threadId/messages returns 500, ask the platform whether it lists that thread. Not listed → 200 {"messages": []}.

Only a thread the platform does not list is treated as empty. A 500 for a thread that does exist is a real failure — an outage, a bad key — and stays a 500, because answering it with empty history would tell the browser the conversation is gone and invite it to start over. Same for a failed listing, or a request with no agentId to list against.

The middleware wraps the handler rather than being added with handler.use: Hono matches middleware only against routes declared after it, and the handler arrives with all of its own already registered.

Test plan

server/src/copilot.test.ts covers the decision over a stub runtime:

• a minted thread the platform does not list reads as empty
• a 500 on a thread that exists stays a 500
• a 500 stays a 500 when the listing itself fails
• a 500 stays a 500 with no agentId to list against

Verified against a live deployment: minted thread → {"messages":[]}, existing thread → 200 with history intact. typecheck and biome check clean.