Description

Improves the SlidingWindowConversationManager truncation strategy to reduce
context size more gracefully without misleading the agent.

• Partially truncate large tool results (>400 chars) preserving first/last 200 chars,
  instead of replacing entire content with an error message
• Replace image blocks (top-level and inside toolResult) with descriptive text placeholders
• Target oldest tool results first so the most relevant recent results are preserved longer
• Add ValueError for invalid per_turn values (0 or negative integer)
• Rename _find_last_message_with_tool_results → _find_oldest_message_with_tool_results
  to accurately reflect iteration order

Related Issues

Closes #1545

Documentation PR

N/A

Type of Change

Other: Enhancement - improve existing truncation behavior

Testing

• I ran hatch run prepare

Added unit tests covering partial truncation, image block replacement,
oldest-first ordering, idempotency guard, and boundary conditions.

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published