Bring Anthropic usechat backend to OpenAI-level tool-use parity and stream robustness

[Copilot]

This updates pkg/aiusechat/anthropic from partial implementation to full backend parity for core tool-use orchestration and stream behavior. The main gaps were unimplemented tool lifecycle methods, missing persisted tool-use UI state, and weaker disconnect/error handling versus the OpenAI backend.

• Tool-use lifecycle parity (critical path)

  • Implemented Anthropic backend support for:
    • UpdateToolUseData
    • RemoveToolUseCall
    • GetFunctionCallInputByToolCallId
  • Wired pkg/aiusechat/usechat-backend.go to call Anthropic implementations instead of stubs.
  • Added Anthropic run-step nil-message guard so nil responses are not wrapped into []GenAIMessage{nil}.

• Persisted tool-use state in Anthropic native messages

  • Added internal ToolUseData storage on Anthropic tool_use blocks.
  • Ensured internal-only fields are stripped before API requests via Clean().

• UI conversion parity for reloaded history

  • Extended ConvertToUIMessage() to emit data-tooluse parts when tool-use metadata exists, in addition to tool-{name} parts.

• Streaming UX parity for tool argument deltas

  • Added aiutil.SendToolProgress(...) calls during:
    • input_json_delta (incremental updates)
    • content_block_stop for tool_use (final update)

• Disconnect/stream robustness

  • Added sse.Err() checks in event handling and decode-error path.
  • Added partial-text extraction on client disconnect and deterministic ordering of partial blocks.
  • Cleans up completed blocks from in-flight state to avoid duplicate partial extraction.

• Correctness + hygiene alignment

  • Continuation model checks now use AreModelsCompatible(...) (instead of strict string equality).
  • Added hostname sanitization in Anthropic error paths (HTTP error parsing and httpClient.Do failures).
  • Replaced unconditional Anthropic debug log.Printf calls with logutil.DevPrintf.

• Targeted coverage additions

  • Added Anthropic tests for:
    • function-call lookup by tool call id
    • tool-use data update + removal
    • data-tooluse UI conversion behavior

// usechat-backend.go
func (b *anthropicBackend) RunChatStep(...) (..., []uctypes.GenAIMessage, ...) {
    stopReason, msg, rateLimitInfo, err := anthropic.RunAnthropicChatStep(ctx, sseHandler, chatOpts, cont)
    if msg == nil {
        return stopReason, nil, rateLimitInfo, err
    }
    return stopReason, []uctypes.GenAIMessage{msg}, rateLimitInfo, err
}

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