Kimi K3 injects tools mid-conversation via a system message that carries
a `tools` array. `dto.Message` had no such field, so the tools were
silently dropped during the parse/re-marshal round trip and the upstream
rejected the request with `'tool_choice'='required' requires a 'tools'
field`.
- add `Message.Tools` (json.RawMessage passthrough)
- omit the `content` key only for tool-loading messages with nil content,
as Kimi rejects `tools` next to `content`; all other messages keep
emitting `"content": null`
- count message-level tools in token estimation
- skip tool-loading messages in channel system prompt injection and make
the compatible handler reuse applySystemPromptIfNeeded
- add kimi-k3 to the moonshot model list
Fixes #7235
