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

[xinkuleee]

Summary

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

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 (first-turn creation is expected there).

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 and full go test ./... pass. Core
(uv run pytest) 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 (lookupOngoingTurnID /
  replayCachedResponses) still require Redis and aren't unit-tested here;
  s.cache is a concrete *redis.Client, so covering them would need miniredis
  or a cache interface — flagged as a possible follow-up.
• Files changed: chat.go (the fix) and chat_test.go (the test), 2 files total.