fix(coderd/x/chatd): stream compaction summaries - #28886

Chats could get permanently stuck once their context crossed the compaction threshold on adaptive-thinking Claude models: every compaction attempt failed with compaction produced an empty summary, and both automatic and manual compaction take the same path, so the chat could never recover.

Problem

compactionSummaryCall deliberately set MaxOutputTokens = nil because the summary request was non-streaming and the Anthropic SDK rejects non-streaming requests whose max_tokens implies a completion longer than ten minutes. But a nil cap makes fantasy's Anthropic provider silently default to 4096 output tokens, and adaptive-thinking Claude models can spend that entire budget on a thinking block, leaving zero summary text.

Fix

• Stream the compaction summary request. Streaming lifts the Anthropic SDK's non-streaming duration restriction, which was the only reason the cap was nil'd.
• Keep the resolved output cap (32k default) on the summary call instead of nil'ing it.
• When the summary comes back empty with truncation evidence (length finish reason, or reasoning with no text), fail with compaction summary was truncated at the output token cap instead of the generic empty-summary error, so the next occurrence is attributable at a glance.

Unit tests cover text-block joining and the truncation classification; the compaction e2e fixtures now serve streamed summaries and assert the summary request streams with max_tokens: 32000. Dogfood UAT on a deployment built from this branch verified manual compaction with Claude Opus 5 (adaptive thinking, twice, with reload persistence) and an OpenAI regression chat.

🤖 Opened by Xum on behalf of @ibetitsmike.