This allows the stats to be written to disk as JSON and analyzed.
- Add dataclasses to hold stats.
- Move stats pretty-print logic to `__str__` of the new `InvocationStatsSummary` class.
- Add `get_stats` and `dump_stats` methods to `InvocationStatsServiceBase`.
- `InvocationStatsService` now throws if stats are requested for a session it doesn't know about. This avoids needing to do a lot of messy null checks.
- Update `DefaultInvocationProcessor` to use the new stats methods and suppresses the new errors.
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