fix(bedrock): reorder reasoning blocks first in assistant messages for multi-turn thinking

[giulio-leone]

Problem

When extended thinking is enabled and a session manager persists/reloads conversation history, assistant messages may have reasoningContent blocks in wrong order. Bedrock strictly requires that if an assistant message contains any thinking blocks, the first block must be thinking:

botocore.errorfactory.ValidationException: If an assistant message contains
any thinking blocks, the first block must be thinking. Found text

This breaks multi-turn conversations when session managers serialize and deserialize messages without preserving block order.

Root Cause

_format_bedrock_messages() formats and filters content blocks but does not enforce Bedrock's ordering constraint. When messages come from external session managers (e.g., AgentCoreMemorySessionManager), blocks may arrive in arbitrary order.

Fix

Added defensive reordering in _format_bedrock_messages(): after formatting each assistant message's content blocks, check if reasoning blocks exist but are not at the beginning. If so, move all reasoningContent blocks before other blocks.

The reordering only triggers when needed (reasoning blocks exist but aren't first), so there's zero overhead for non-thinking conversations.

Tests

• Added test_format_bedrock_messages_reorders_reasoning_blocks_first that verifies blocks are correctly reordered when reasoningContent appears after text in an assistant message
• All 124 existing Bedrock tests pass

Fixes #1698

[giulio-leone]

Friendly ping — CI is green, tests pass, ready for review whenever convenient. Happy to address any feedback. Thanks! 🙏

[giulio-leone]

Friendly ping — this branch has been refreshed on the latest upstream base and all current review feedback has been addressed. It should be ready for review whenever you have a chance. Happy to make any further changes quickly.