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
Anthropic does not support assistant-message prefill and returns a 400
error when the conversation ends with an assistant turn. This commonly
happens when heartbeat/system messages accumulate trailing assistant
replies in the session history.
The _merge_consecutive method already handles same-role merging but did
not strip trailing assistant messages. The base provider's
_enforce_role_alternation (used by OpenAI-compat) does strip them, but
AnthropicProvider uses its own _merge_consecutive instead.
Add a trailing-assistant stripping loop to _merge_consecutive, matching
the behavior already present in _enforce_role_alternation.
Includes 7 new tests covering merge + strip behavior.
