fix: normalize chat completions system messages

[MoeMion]

Root cause

Chat-completions-compatible providers can reject or inconsistently interpret requests containing multiple system messages at different positions. File context, tool guidance, and final synthesis instructions could therefore produce an invalid or ambiguous message sequence.

Impact

OpenAI Chat Completions and OpenRouter Chat requests are affected. Non-system messages, including tool calls and tool results, keep their original order and payload.

Changes

• Merge all non-empty system message content into one leading system message, separated by blank lines.
• Preserve the order of all non-system messages.
• Add regression tests for OpenAI Chat Completions and OpenRouter Chat, including tool-call payload preservation.

Validation

• go test ./internal/infra/llm
• go vet ./internal/infra/llm
• gofmt -d backend/internal/infra/llm/openai_chat_completions.go backend/internal/infra/llm/request_params_test.go
• git diff --check upstream/dev...HEAD

Full make test was attempted, but unrelated SQLite packages could not compile in the local Windows CGO environment because sqlite3.h is unavailable.

[MoeMion]

API returned

{
  "error": {
    "code": "400",
    "message": "System message must be at the beginning.",
    "type": "BadRequestError"
  }
}

when system prompt not at beginning.

[github-actions]

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