refactor(stream): move stream contracts out of adapters

[limityan]

Summary

• Move provider-neutral stream DTOs and tool-call accumulation ownership into bitfun-agent-stream.
• Keep bitfun-ai-adapters compatibility re-exports and add GeminiUsage / UnifiedTokenUsage conversions.
• Move provider stream fixtures to ai-adapters and sync concise layer ownership docs.

Risk / impact

• No product behavior change intended; provider wire parsing remains in ai-adapters, while replay and tool-call accumulation are owned by agent-stream.
• Existing bitfun_ai_adapters::{UnifiedResponse, UnifiedTokenUsage, UnifiedToolCall} and tool_call_accumulator paths remain available through re-exports.
• The change removes the execution -> adapters reverse dependency and keeps tests with the provider parsing owner.

Verification

• git diff --cached --check
• node --test scripts/check-core-boundaries.test.mjs
• node scripts/check-core-boundaries.mjs
• layer dependency metadata check: 38 workspace layer edges, 0 reverse-layer dependency violations
• cargo test -p bitfun-agent-stream
• cargo test -p bitfun-ai-adapters
• cargo check -p bitfun-core --no-default-features
• cargo check -p bitfun-core --features product-full
• cargo check --workspace
• pnpm --dir src/web-ui run test:run src/shared/services/reviewTargetClassifier.test.ts src/shared/services/review-team/pathMetadata.test.ts src/shared/services/review-team/risk.test.ts src/shared/services/reviewTeamService.test.ts
• pnpm run check:repo-hygiene