Extend `_merge_consecutive` so the three invariants from
`LLMProvider._enforce_role_alternation` all hold for Anthropic:
1. collapse consecutive same-role turns (unchanged)
2. no trailing assistant — Anthropic rejects prefill (unchanged)
3. no leading assistant — Anthropic requires the first turn be user
4. non-empty messages array — recover the last stripped assistant as a
user turn when every turn got stripped, so callers don't hit a
secondary "messages array empty" 400
Anthropic-specific wrinkle: `tool_use` blocks live inside `content` (not
a separate `tool_calls` field) and are illegal inside user turns, so
both recovery paths skip any message carrying them rather than silently
producing a malformed request.
Adds 4 unit tests covering the new branches, including the tool_use
opt-outs, and updates the existing `test_single_assistant_stripped` to
reflect the new rerouting contract.
Made-with: Cursor
Xubin RenandXubin Ren
009cce78ad