Normalize api_messages before each API call for consistent prefix
matching across turns:
1. Strip leading/trailing whitespace from system prompt parts
2. Strip leading/trailing whitespace from message content strings
3. Normalize tool-call arguments to compact sorted JSON
This enables KV cache reuse on local inference servers (llama.cpp,
vLLM, Ollama) and improves cache hit rates for cloud providers.
All normalization operates on the api_messages copy — the original
conversation history in messages is never mutated. Tool-call JSON
normalization creates new dicts via spread to avoid the shallow-copy
mutation bug in the original PR.
Salvaged from PR #7875 by @waxinz with mutation fix.
