fix(llm): narrow raw overlays

[nexxeln]

Summary

• reject raw http.body overlays that target protocol-owned body roots like model, messages, tools, system, generation, and streaming fields
• keep raw overlays available for non-protocol provider fields
• tighten prepare coverage for deterministic query order, auth winning over header overlays, and local invalid-request failures

Tests

• cd packages/llm && bun typecheck
• cd packages/llm && bun test
• pre-push hook passed: bun turbo typecheck (29/29 tasks)

Note

• local worktree has an unrelated unstaged formatting change in packages/llm/src/schema/options.ts; it is not part of this PR