fix(session): decode stored structured output format on every message publish

[filipeforattini]

Summary

• fix(provider): complete support for Claude Opus 5.5 and GPT-6 #369 fixed Expected OutputFormatJsonSchema when a promoted prompt republished its stored format, which reads back from the database as plain JSON. The same failure hit every other place that republishes a stored user message: compaction replay (compaction.ts copied original.format), Session.fork (spreads msg.info) and the diff summary (summary.ts republishes target.info).
• The decode now happens once, in Session.updateMessage, the single publisher of MessageUpdated, and the two call-site decodes fix(provider): complete support for Claude Opus 5.5 and GPT-6 #369 added in prompt.ts are removed.
• The core (v2) runtime does not carry a user message format, so nothing changes there.

Tests

• compaction.test.ts: overflow compaction replays a structured output turn read back from the database and keeps its json_schema format.
• session.test.ts: forking a session copies a structured output turn with its format.

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