fix(ai-gateway): emit finish_reason before [DONE] for Pi streaming

[Anshgrover23]

Description:

fixes the gateway stream shape so Pi chat/pipes stop failing after auth succeeds.

• before:

Fix: Pi expects an OpenAI-style terminal chunk with finish_reason before data: [DONE]; Anthropic and OpenAI adapters only sent content/usage, so clients threw Stream ended without finish_reason on Claude and GPT models.

Test passing:

Files changed:

• packages/ai-gateway/src/providers/anthropic.ts — map stop_reason → OpenAI finish_reason; emit terminal chunk; add network_error on stream errors
• packages/ai-gateway/src/providers/openai.ts — capture upstream finish_reason, emit terminal chunk before [DONE]; fix stream typing + ChatCompletionMessageParam
• packages/ai-gateway/src/handlers/models.ts — remove duplicate curated gpt-5.5 / gpt-5.4 / gpt-5.4-mini entries
• packages/ai-gateway/src/test/anthropic-proxy.unit.test.ts — streaming finish_reason + tool_calls tests
• packages/ai-gateway/src/test/openai-models.unit.test.ts — streaming finish_reason test + TS fixes

[Anshgrover23]

@louis030195 Can I get a review on this one ?