Add durable settlement to LangChain reserve modes

[amavashev]

Summary

• route model and tool reserve modes through the Python SDK's managed heartbeat and durable settlement path
• preserve known spend after provider or tool execution and expose settlement failures consistently
• normalize provider usage, including OpenAI and Anthropic prompt-cache accounting
• bound deterministic settlement keys and remove the stale duplicated docs page

Verification

• 187 tests passed; 99.32% coverage
• Ruff and mypy clean
• sdist and wheel build clean

Dependency

Depends on runcycles/cycles-client-python#95 and the runcycles 0.5.3 release.