docs: add streaming session events field-level reference guide (#717)

Add docs/guides/streaming-events.md documenting every session event type
with its exact data payload fields, required vs optional status, and
descriptions. Includes:

- Per-event field tables for all ~40 event types
- Ephemeral vs persisted classification
- Agentic turn flow diagram showing event ordering
- Quick reference summary table
- Multi-language subscription examples (TS, Python, Go, .NET)
- Permission request discriminated union breakdown
- Notes on SDK type differences (unified Data vs per-event types)

Addresses user request for a field mapping so developers don't have to
read SDK source code to discover which fields are populated on which
event types.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: patniko