feat: emit contextWindowFallback stream event before context reduction

[Zelys-DFKH]

Description

When the agent handles a context window overflow, it calls `reduce_context()` synchronously. This is a blocking operation that can take a noticeable amount of time in long sessions, but callers have no way to know it has started. From outside, the stream just goes quiet until the reduction finishes and the model is called again.

This change adds a `contextWindowFallback` event emitted immediately before `reduce_context()` is called, giving streaming consumers a real-time signal that context compression is starting.

Changes:

• Add `ContextWindowFallbackEvent` TypedDict to `streaming.py` (alongside the other streaming event types)
• Add `ContextWindowFallbackStreamEvent` TypedEvent subclass in `_events.py` so the event follows the same protocol as all other agent events
• Yield the new event from the `except ContextWindowOverflowException` handler in `agent.py`, before the `reduce_context()` call

The `contextWindowFallback` key is also added to `StreamEvent` so consumers can pattern-match it in the same way as `messageStart`, `messageStop`, and the other stream event types.

Related Issues

Fixes #1511

Documentation PR

N/A

Type of Change

New feature

Testing

• Added `test_stream_async_emits_context_window_fallback_event`, which mocks `reduce_context` to isolate event emission and verifies exactly one fallback event appears in the stream with the overflow message in the payload
• All 120 existing agent tests pass

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

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