fix(deepagents): extract text from content blocks when building the summary (#739)

## Description

`SummarizationMiddleware`'s `createSummary` falls back to
`JSON.stringify(response.content)` when the summarization model's
response content is not a string:

```ts
return typeof response.content === "string"
  ? response.content
  : JSON.stringify(response.content);
```

When the summarization model responds with a content block array, the
raw blocks are stringified verbatim into the summary `HumanMessage`.
This happens in practice when:

- **Thinking/reasoning is enabled** on the summarization model (e.g.
Claude on Bedrock with `thinking`): the content is `[{type: "reasoning",
reasoning: ..., signature: <opaque multi-KB signature>}, {type: "text",
...}]`, and the reasoning signature lands in the summary.
- **`invoke()` takes the internal streaming path** (a `prefersStreaming`
callback handler is attached, e.g. inside `streamEvents`, and
`disableStreaming` is not set): the aggregated `AIMessageChunk` content
is an array of many small text fragments.

Observed in production: a summary message of ~56KB consisting of an ~8KB
opaque reasoning signature plus hundreds of
`{"type":"text","text":"..."}` fragments — the "summary" ended up
bloating the context it was supposed to compact, and subsequent model
calls overflowed.

## Fix

Use the message's `text` accessor for non-string content, which extracts
and joins only the text blocks. String content is returned as-is
(unchanged behavior).

## Testing

Added a unit test where the summarization model returns reasoning +
split text blocks, asserting the summary message contains the joined
text and no reasoning payload. All 33 existing tests pass.

---------

Co-authored-by: Hunter Lovell <hunter@hntrl.io>