Improve session cost & context usage UX

[ethanndickson]

• Session cost is wholly incorrect (🤖 fix: correct context usage display for multi-step tool calls #893, 🤖 fix: preserve usage data when stream is interrupted #837, 🤖 fix: accurate cost estimation for multi-step tool usage #831, 🤖 fix: include step providerMetadata in liveUsage for cache creation display #864)
• Context usage & session cost does not update mid-stream (🤖 feat: mid-stream token usage updates #757)
• Session cost is not calculated accurately when switching models within a session.
• Partial session cost is lost when editing messages.
• Partial/full session cost is lost when truncating/clearing history.