fix: group multiple Bedrock tool results into single user message

[atian8179]

Problem

When a Bedrock model makes multiple tool calls in a single response, CrewAI sends each tool result as a separate user message:

Message 0: user (prompt)
Message 1: assistant (toolUse A, toolUse B)
Message 2: user (toolResult A)  ← Separate message
Message 3: user (toolResult B)  ← Separate message — causes ValidationException

Bedrock's Converse API requires all tool results for a given assistant message to be grouped in one user message.

Solution

When converting tool messages to Bedrock format, check if the previous converse message is already a user message containing toolResult blocks. If so, append the new toolResult to it instead of creating a new message:

Message 0: user (prompt)
Message 1: assistant (toolUse A, toolUse B)
Message 2: user (toolResult A, toolResult B)  ← Grouped correctly

Fixes #4749

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Note

Medium Risk
Moderate risk because it changes how tool responses are serialized into the Bedrock conversation history, which can affect multi-tool-call flows and message alternation edge cases.

Overview
Prevents Bedrock Converse ValidationException when a model returns multiple tool calls in one assistant turn by grouping all corresponding toolResult blocks into a single subsequent user message.

Updates _format_messages_for_converse so consecutive tool role messages append their toolResult blocks onto the previous user message (when it already contains tool results) instead of emitting multiple separate user turns.

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[greysonlalonde]

Superseded by #4775, merged 2026-03-10. Closing.