fix: ensure assistant messages have reasoningContent when thinking is enabled

[giulio-leone]

Issue

Closes #1698

Problem

When extended thinking is enabled, Bedrock requires every assistant message to start with a reasoningContent block. Custom session managers (e.g., AgentCoreMemorySessionManager) or external history providers may strip these blocks during serialization/deserialization, causing a ValidationException on subsequent API calls:

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

Root Cause

The SDK correctly constructs reasoningContent blocks during streaming and preserves them through the event loop. However, custom session managers that serialize/deserialize conversation history may strip reasoningContent blocks, breaking Bedrock's multi-turn thinking constraint.

Solution

Added defensive handling in _format_bedrock_messages() to ensure assistant messages conform to Bedrock's thinking requirements:

1. Missing reasoningContent: When thinking is enabled and an assistant message has no reasoningContent block, injects a minimal redactedContent placeholder at position 0
2. Misordered blocks: When reasoningContent exists but is not the first content block, reorders blocks to put reasoning first

This is a defensive measure — it handles misbehaving session managers without breaking correct flows where reasoningContent is already properly preserved.

Testing

Added 6 new tests covering:

• test_format_request_injects_reasoning_when_thinking_enabled_and_missing — placeholder injection when thinking blocks are stripped
• test_format_request_no_injection_when_thinking_disabled — no modification without thinking
• test_format_request_preserves_existing_reasoning_when_thinking_enabled — existing blocks preserved
• test_format_request_reorders_reasoning_to_first_when_thinking_enabled — block reordering
• test_format_request_multi_turn_thinking_session_reload — multi-turn session reload scenario
• test_format_request_does_not_inject_reasoning_for_user_messages — user messages untouched

All 131 tests pass (125 existing + 6 new).

[giulio-leone]

Friendly ping — ensures assistant messages include reasoningContent when thinking mode is enabled, preventing missing reasoning context in multi-turn conversations.