fix(apicompat): repair tool_use/tool_result pairing on the Responses→Anthropic path

[visa2]

Problem

When an OpenAI Chat Completions client targets an Anthropic-platform group, ForwardAsChatCompletions converts the request CC → Responses → Anthropic (ChatCompletionsToResponses → ResponsesToAnthropicRequest) before forwarding it upstream. The same ResponsesToAnthropicRequest is used by the /v1/responses → Anthropic path.

That converter emits each function_call as its own assistant message and each function_call_output as its own user message, relying solely on mergeConsecutiveMessages to alternate roles. That is not enough to satisfy Anthropic's tool-pairing invariants, so a trimmed or partial tool history produces an upstream 400:

Failed to deserialize / 400: tool_use_id found in tool_result blocks: call_00_...
Each tool_result block must have a corresponding tool_use block in the previous message.

Failure classes left unrepaired:

• orphan tool_result — a client doing sliding-window context management keeps a recent tool result but drops the assistant tool_calls message that announced it, so the tool_result has no matching tool_use;
• unanswered / dangling tool_use — a parallel call whose sibling result never came back, or a call left dangling, which Anthropic also rejects.

Approach

Add normalizeAnthropicToolPairing, run between two merge passes:

1. first merge groups parallel calls and their results so the pairing pass sees them together;
2. the pairing pass indexes every tool_result by its tool_use id, keeps only answered tool_use blocks (dropping unanswered/dangling calls, and the assistant message entirely when nothing else remains), and re-emits the matching tool_result blocks as the immediately following user message; standalone/orphan tool_results are dropped from their original position;
3. the second merge restores alternation.

This mirrors normalizeChatMessages on the Responses→Chat path.

Tests

• responses_to_anthropic_tool_pairing_test.go — repair on direct Responses input (developer message between call and output, parallel both-answered kept grouped, parallel one-unanswered dropped, orphan tool_result, dangling call, single-call baseline).
• responses_to_anthropic_cc_chain_test.go — drives the real ChatCompletionsToResponses → ResponsesToAnthropicRequest chain and reproduces the production 400 (orphan and unanswered-parallel). Both fail without the repair and pass with it.

go build ./..., the full apicompat suite, and golangci-lint run ./... (v2.9) are all green.

Note on overlap

This touches responses_to_anthropic_request.go, which #2916 also modifies (system from instructions/developer, polymorphic arguments/output, tool schema). The changes are orthogonal — #2916 does not add any tool_use/tool_result pairing repair — but depending on merge order one side may need a trivial rebase.