fix(anthropic): merge parallel tool results, ordinal stream tool indices, array system content, strict stream decode

Review round 1 fixes (1 Critical + 5 Important) on the anthropic
adapter/translator.

- CRITICAL: consecutive role:"tool" messages each became their own
  Anthropic user turn. Anthropic requires alternating user/assistant
  turns and 400s on two consecutive same-role messages; parallel tool
  calls (one assistant message with N tool_calls followed by N
  role:"tool" messages) are the standard agentic transcript shape.
  anthropicRequestFromOpenAI now accumulates consecutive tool messages
  into one Anthropic user turn carrying all of their tool_result
  blocks (anthropicToolResultBlock replaces anthropicToolResultMessage).
  Added chat_parallel_tool_results fixture.
- Streaming tool_calls[].index passed Anthropic's raw content-block
  index through, so a tool_use block after a text block emitted
  index:1 instead of the required contiguous 0-based ordinal.
  anthropicStreamState now tracks a per-stream tool-call ordinal
  counter plus a content-block-index -> ordinal map, populated on
  content_block_start and consulted on input_json_delta. Added the
  stream_text_then_tool fixture (text block at index 0, tool_use at
  index 1) proving the emitted index is 0.
- A system message with array-form content ([{"type":"text",...}])
  was silently dropped (only a string content was handled). Added
  systemTextFromContentParts: text parts join with "\n"; a non-text
  part returns a *translateError instead of being dropped. Added
  chat_system_array_content (success) and
  chat_system_array_content_error fixtures.
- Malformed JSON on a recognized stream event (message_start,
  content_block_start, content_block_delta, message_delta) was
  swallowed as (nil, nil), which could silently corrupt a client's
  reassembled tool-call arguments. All four now return an error
  wrapping errUpstream; forwardStream already propagates that and
  withholds the terminal [DONE]. The unrecognized-event-type and
  error-event branches are unchanged. Added
  TestAnthropicStreamState_MalformedEventReturnsError.
- providerAdapter.injectAuth's doc comment claimed a universal
  empty-key no-op; updated to state the per-type divergence
  (openai-type: keyless is a no-op; anthropic-type: the constructor
  rejects an empty key, so injectAuth is never called with one).
- Added case-insensitive-Content-Type coverage for both adapters'
  streaming-vs-JSON gate (previously untested): one subtest each in
  provider_openai_test.go / provider_anthropic_test.go using a fake
  upstream that sends "Text/Event-Stream; charset=utf-8", asserting
  the streaming path (not the JSON fallback) is taken.

Gate: go test ./... -count=1 and -race both green; gofmt, go vet,
fieldalignment, golangci-lint (incl. gosec), govulncheck all clean.

translate_anthropic_test.go

@@ -270,6 +270,16 @@ func TestAnthropicStreamGolden_ToolUse(t *testing.T) {
 	runStreamGolden(t, "stream_tooluse")
 }
 
+// TestAnthropicStreamGolden_TextThenTool covers a text block followed by
+// a tool_use block (Anthropic content-block index 0 then 1) and proves
+// the emitted tool_calls[0].index is the 0-based tool-call ordinal (0),
+// not the raw Anthropic content-block index (1) — review fix: OpenAI's
+// contract requires tool_calls[].index to be contiguous from 0 over the
+// tool-call array, regardless of how many non-tool_use blocks preceded it.
+func TestAnthropicStreamGolden_TextThenTool(t *testing.T) {
+	runStreamGolden(t, "stream_text_then_tool")
+}
+
 // TestAnthropicStreamState_ErrorEvent proves an "error" SSE event
 // translates to one OpenAI-shaped {"error":{...}} chunk and a non-nil
 // terminal error, per the stream mapping table's error row — built
@@ -307,6 +317,33 @@ func TestAnthropicStreamState_ErrorEvent(t *testing.T) {
 	}
 }
 
+// TestAnthropicStreamState_MalformedEventReturnsError proves a malformed
+// (invalid-JSON) data payload on each recognized event type returns an
+// error wrapping errUpstream, rather than being swallowed as (nil, nil) —
+// review fix: a dropped input_json_delta would otherwise silently corrupt
+// the client's reassembled tool-call arguments. The "error" event type
+// itself already returned an error before this fix and is covered by
+// TestAnthropicStreamState_ErrorEvent, not repeated here.
+func TestAnthropicStreamState_MalformedEventReturnsError(t *testing.T) {
+	const corrupt = `{"type":"message_start","message":{`
+
+	for _, event := range []string{"message_start", "content_block_start", "content_block_delta", "message_delta"} {
+		t.Run(event, func(t *testing.T) {
+			st := newAnthropicStreamState(goldenGatewayModel, goldenCreated)
+			chunks, err := st.translate(sseEvent{event: event, data: []byte(corrupt)})
+			if err == nil {
+				t.Fatalf("translate(%s, corrupt data): want error, got nil (chunks=%v)", event, chunks)
+			}
+			if !errors.Is(err, errUpstream) {
+				t.Errorf("translate(%s, corrupt data): err = %v, want errors.Is(err, errUpstream)", event, err)
+			}
+			if len(chunks) != 0 {
+				t.Errorf("translate(%s, corrupt data): chunks = %v, want none", event, chunks)
+			}
+		})
+	}
+}
+
 // TestAnthropicListModels_404NotSpeciallyHandled proves listModels
 // returns a plain *providerHTTPError on a 404 (an older Anthropic API
 // version that predates the Models endpoint) rather than any special