feat: add response_format json_object support

[lukasBlckIT]

Problem

When response_format: {"type": "json_object"} is set, kiro-gateway accepts the parameter but does not forward it to the Kiro API. Claude responds with markdown-wrapped JSON (json ... ) instead of raw JSON, causing JSON parsing failures in downstream clients (e.g. ValidationError in Honcho).

Solution

Two-layer fix:

1. System prompt injection (kiro/converters_openai.py) — when response_format: json_object is detected, appends a JSON instruction to the system prompt telling Claude to respond with raw JSON only, no markdown wrappers.

2. Markdown stripping (kiro/routes_openai.py + new kiro/json_response_format.py) — post-processes non-streaming responses to strip any remaining markdown code block wrappers as a fallback.

Behaviour

• response_format: json_object → raw, parseable JSON guaranteed
• response_format: text or no response_format → completely unaffected
• Streaming: system prompt injection is sufficient per OpenAI spec (chunks are text deltas, not full responses)
• Existing system prompts are preserved and the JSON instruction is appended

Testing

25/25 test cases passed locally, covering non-streaming, streaming, Swedish unicode characters, tool calling, auth errors, and edge cases.

[cla-bot]

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[cla-bot]

Thanks for the PR! 🎉

Before merge, we need a one-time CLA confirmation.
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[lukasBlckIT]

I have read the CLA and I accept its terms