fix: use correct slice index for Bedrock tool result coalescing (#2237)

**Description**

Fix two bugs in the Anthropic-to-Bedrock translator message merging
logic at `internal/translator/anthropic_awsbedrock.go`.

**Bug 1: Wrong slice index (original report)**

When system messages are present in the request body,
`promoteAnthropicSystemMessagesToParam` filters them out, producing a
shorter `messages` slice. The coalescing loop was using
`body.Messages[i+1]` (the original unfiltered slice) instead of
`messages[i+1]` (the filtered slice), causing index misalignment that
could read wrong messages or panic with out-of-bounds access.

**Bug 2: Coalescing logic was unreachable (root cause)**

Tool result messages have `Role: "user"`, so they match `case
anthropicschema.MessageRoleUser` and go through `convertUserMessage()` —
which handles tool results individually but never coalesces consecutive
ones. The coalescing logic in the `default` branch was dead code for
tool results since they never reach that branch.

**Changes**

- Moved the `hasToolResult` check + coalescing loop into the
`MessageRoleUser` case branch
- Non-tool-result user messages still go through `convertUserMessage()`
- Changed `body.Messages[i+1]` → `messages[i+1]` to use the correct
filtered slice
- Simplified the `default` branch to just return an error for unexpected
roles
- Updated existing test to expect coalesced output (3 messages instead
of 4)
- Added regression test with system messages to verify index alignment

**Testing**

- All existing translator tests pass
- New
`TestAnthropicToAWSBedrockTranslator_RequestBody_ToolResultMessagesWithSystemMessages`
verifies the fix works when system messages are present alongside
consecutive tool results

---------

Signed-off-by: liuhy <liuhongyu@apache.org>
Co-authored-by: Ignasi Barrera <ignasi@tetrate.io>

Author: Aias00

.gitignore

@@ -52,3 +52,4 @@ inference-extension-conformance-test-report.yaml
 /aigw
 site/.cursorrules
 /.cursor
+.worktrees/

internal/translator/anthropic_awsbedrock.go

@@ -78,36 +78,29 @@ func (a *anthropicToAWSBedrockTranslator) RequestBody(_ []byte, body *anthropics
 		msg := &messages[i]
 		switch msg.Role {
 		case anthropicschema.MessageRoleUser:
-			bedrockMsg, convErr := a.convertUserMessage(msg)
-			if convErr != nil {
-				return nil, nil, convErr
-			}
-			bedrockReq.Messages = append(bedrockReq.Messages, bedrockMsg)
-			i++
-		case anthropicschema.MessageRoleAssistant:
-			bedrockMsg, convErr := a.convertAssistantMessage(msg)
-			if convErr != nil {
-				return nil, nil, convErr
-			}
-			bedrockReq.Messages = append(bedrockReq.Messages, bedrockMsg)
-			i++
-		default:
-			// Check for tool_result content blocks (these come as "user" role in Anthropic
-			// but we handle them specially).
 			if hasToolResult(msg) {
 				bedrockMsg := a.convertToolResultMessage(msg)
 				// Coalesce consecutive tool result messages.
-				for i+1 < msgLen && hasToolResult(&body.Messages[i+1]) {
-					nextMsg := &body.Messages[i+1]
+				for i+1 < msgLen && hasToolResult(&messages[i+1]) {
+					nextMsg := &messages[i+1]
 					nextBedrockMsg := a.convertToolResultMessage(nextMsg)
 					bedrockMsg.Content = append(bedrockMsg.Content, nextBedrockMsg.Content...)
 					i++
 				}
 				bedrockReq.Messages = append(bedrockReq.Messages, bedrockMsg)
 			} else {
-				return nil, nil, fmt.Errorf("%w: unexpected role: %s", internalapi.ErrInvalidRequestBody, msg.Role)
+				bedrockMsg, convErr := a.convertUserMessage(msg)
+				if convErr != nil {
+					return nil, nil, convErr
+				}
+				bedrockReq.Messages = append(bedrockReq.Messages, bedrockMsg)
 			}
 			i++
+		case anthropicschema.MessageRoleAssistant:
+			bedrockReq.Messages = append(bedrockReq.Messages, a.convertAssistantMessage(msg))
+			i++
+		default:
+			return nil, nil, fmt.Errorf("%w: unexpected role: %s", internalapi.ErrInvalidRequestBody, msg.Role)
 		}
 	}
 
@@ -236,13 +229,13 @@ func (a *anthropicToAWSBedrockTranslator) convertUserMessage(msg *anthropicschem
 	return bedrockMsg, nil
 }
 
-func (a *anthropicToAWSBedrockTranslator) convertAssistantMessage(msg *anthropicschema.MessageParam) (*awsbedrock.Message, error) {
+func (a *anthropicToAWSBedrockTranslator) convertAssistantMessage(msg *anthropicschema.MessageParam) *awsbedrock.Message {
 	bedrockMsg := &awsbedrock.Message{Role: awsbedrock.ConversationRoleAssistant}
 	if msg.Content.Text != "" {
 		bedrockMsg.Content = []*awsbedrock.ContentBlock{
 			{Text: ptr.To(msg.Content.Text)},
 		}
-		return bedrockMsg, nil
+		return bedrockMsg
 	}
 	bedrockMsg.Content = make([]*awsbedrock.ContentBlock, 0, len(msg.Content.Array))
 	for i := range msg.Content.Array {
@@ -277,7 +270,7 @@ func (a *anthropicToAWSBedrockTranslator) convertAssistantMessage(msg *anthropic
 			})
 		}
 	}
-	return bedrockMsg, nil
+	return bedrockMsg
 }
 
 func (a *anthropicToAWSBedrockTranslator) convertToolResultMessage(msg *anthropicschema.MessageParam) *awsbedrock.Message {