fix(plugins): hide prose before structured stream envelopes

[lalalune]

Summary

• stop forwarding ordinary text-delta frames from OpenAI streamStructured fullStream into the visible structured stream
• suppress Eliza Cloud SSE delta.content prose while the structured Stage-1 reply envelope is streamed from tool-call argument deltas
• add mixed-order regression coverage for both OpenAI and Eliza Cloud so leading prose cannot force plaintext passthrough and expose raw JSON

Verification

• bun run --cwd plugins/plugin-openai test __tests__/streamstructured-tool-input-delta.shape.test.ts
• bun run --cwd plugins/plugin-elizacloud test __tests__/text-streaming.test.ts
• bun run --cwd plugins/plugin-openai typecheck
• bun run --cwd plugins/plugin-elizacloud typecheck
• bunx @biomejs/biome check plugins/plugin-openai/models/text.ts plugins/plugin-openai/__tests__/streamstructured-tool-input-delta.shape.test.ts plugins/plugin-elizacloud/src/models/text.ts plugins/plugin-elizacloud/__tests__/text-streaming.test.ts
• git diff --check

Fixes #15531

• N/A - provider streaming bug; no UI surface changed directly

• N/A - provider streaming bug; no UI surface changed directly

• N/A - no rendered workflow change; deterministic stream tests cover the leak path

• 15533-15531-openai-streamstructured-test.txt

• N/A - provider/backend stream shaping only

• 15533-15531-live-trajectory-note.txt

• 15533-15531-elizacloud-text-streaming-test.txt

• N/A - no app UI/OCR surface changed

[claude]

Claude encountered an error —— View job

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