fix: prune stale tool results during session repair

[he-yufeng]

Summary

• prune orphaned or duplicate toolResult blocks while repairing restored session histories
• keep the existing matching results and add only the missing interruption results, so the next user message matches the preceding assistant toolUse blocks
• add a regression test for mixed duplicate/stale/missing tool-result history

Fixes #2296

Tests

• .venv\Scripts\python.exe -m pytest tests\strands\session\test_repository_session_manager.py -q -k "fix_broken_tool_use"
• .venv\Scripts\python.exe -m pytest tests\strands\session\test_repository_session_manager.py -q
• $env:RUFF_CACHE_DIR='.tmp\ruff-cache'; .venv\Scripts\python.exe -m ruff check src\strands\session\repository_session_manager.py tests\strands\session\test_repository_session_manager.py
• $env:RUFF_CACHE_DIR='.tmp\ruff-cache'; .venv\Scripts\python.exe -m ruff format --check src\strands\session\repository_session_manager.py tests\strands\session\test_repository_session_manager.py
• .venv\Scripts\python.exe -m py_compile src\strands\session\repository_session_manager.py tests\strands\session\test_repository_session_manager.py
• git diff --check