fix(conversation): avoid creating a conversation on chat reconnect

[xinkuleee]

Summary

Reconnect previously called ensureConversation (get-or-create). A reconnect
only resumes an in-progress turn, so when a client reconnects to an agent
instance that has no existing conversation, creating one is unnecessary and a
side effect — it leaves a stray empty conversation and then immediately ends the
stream because there is no ongoing turn.

This changes Reconnect to look the conversation up read-only
(conversationRepo.Get). If none exists, it sends the terminal done event and
returns without creating a conversation or touching the ongoing-turn cache.
Chat still uses ensureConversation, where first-turn creation is expected.

Validation

• make precommit-run
• cd backend && go test ./...
• cd core && uv run pytest
• Frontend build/lint, if working from a frontend source checkout
• Not run; explain below

Backend-only change. make precommit-run, go test ./..., and
golangci-lint run all pass locally. Core and frontend suites were not run
because neither is touched by this change.

Checklist

• Linked relevant issues or explained why there is no issue.
• Updated docs and examples where behavior changed.
• Updated CHANGELOG.md under ## [Unreleased] for user-facing changes.
• Regenerated and committed generated files after proto, Wire, or OpenAPI changes.
• Confirmed no secrets, tokens, credentials, or sensitive logs are included.

Notes for reviewers

• New test TestReconnect covers the "no conversation -> don't create, send
  done" path. The cache-backed resume paths still require Redis and aren't
  unit-tested here (s.cache is a concrete *redis.Client); covering them would
  need miniredis or a cache interface — possible follow-up.
• Files changed: chat.go (fix) and chat_test.go (test).
• Branch is rebased onto latest main.

[jasonchen922]

LGTM

[Copilot]

Pull request overview

This PR adjusts the backend conversation chat reconnection flow so that Reconnect no longer performs a get-or-create on conversations. Instead, it performs a read-only lookup and, when no conversation exists, it terminates the SSE stream cleanly by emitting a done event—preventing stray empty conversation records from being created as a side effect of reconnecting.

Changes:

• Updated Service.Reconnect to use conversationRepo.Get (read-only) instead of ensureConversation (get-or-create).
• Added an explicit “no conversation” fast-path that emits a terminal done SSE event and returns without touching the ongoing-turn cache.
• Added a unit test covering the “no conversation -> don’t create one, send done” reconnect behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
backend/internal/biz/conversation/impl/chat.go	Switches reconnect to read-only conversation lookup and cleanly ends the stream when no conversation exists.
backend/internal/biz/conversation/impl/chat_test.go	Adds coverage ensuring reconnect doesn’t create a conversation and only emits a terminal done event when none exists.