Include conversation history in generation inputs

[DavidTraina]

Each generation's input previously contained only the messages since the previous generation, so opening a generation in Langfuse never showed the conversation the model actually received.

This change keeps a running per-session message history and records it as every generation's input:

• User messages (with their tool definitions), assistant messages, and tool results are appended to the history in order.
• Each generation's input is a snapshot of the history at the point the generation starts.
• The history survives the session.idle flush, so generations in later turns include earlier turns.

Scope notes:

• The system prompt is still not included: no event the plugin consumes carries it, so recording it would mean fabricating data. This PR is limited to accumulating message history and tool definitions.
• Compaction is not special-cased: after a compaction, the recorded history still contains the pre-compaction messages rather than the summary the model actually sees from then on.

Tested with pnpm run test:integration: added a test asserting that a later turn's generation input contains the prior turn's user and assistant messages across an idle flush, and updated the existing multi-generation expectations to include the accumulated history.

Addresses the conversation-history part of #24

Opening this as a draft since you mentioned wanting to compare behavior with the Claude and Codex plugins first — happy to adjust the approach to whatever direction you land on.

[CLAassistant]

All committers have signed the CLA.