Streaming ContentThinking chunks lack proper <thinking> tag boundaries

## Current behavior When streaming with `content="all"`, each `ContentThinking` chunk is yielded as an independent object. If you print or concatenate these, each fragment gets its own `<thinking>...</thinking>` wrapper (via `__str__()`): ```python chat = ChatAnthropic(model="claude-sonnet-4-20250514") async for chunk in chat.stream_async("Explain recursion", content="all"): print(chunk, end="") ``` Output: ``` <thinking> The user wants an explanation </thinking> <thinking> of recursion. I should </thinking> <thinking> start with a simple definition... </thinking> Recursion is a technique where... ``` With `content="text"`, the situation is different but also problematic — thinking text is yielded as bare strings indistinguishable from response text: ```python async for chunk in chat.stream_async("Explain recursion", content="text"): print(chunk, end="") ``` Output: ``` The user wants an explanation of recursion. I should start with a simple definition...Recursion is a technique where... ``` No way to tell where thinking ended and the response began. ## Expected behavior The streaming layer should emit `<thinking>` tags at the boundaries — once at the start of thinking, once at the end — so that concatenated output is well-formed: For `content="text"`: ``` <thinking> The user wants an explanation of recursion. I should start with a simple definition... </thinking> Recursion is a technique where... ``` For `content="all"`, the yielded objects stay the same (`ContentThinking` per chunk), but the display/echo output should show proper boundaries. ## Why this matters Downstream consumers (like shinychat) that receive a stream and need to separate thinking from response content currently have to implement their own stateful tracking of `ContentThinking` objects, accumulate the thinking text, and reconstruct the tag boundaries themselves. If the stream already had correct boundaries in text mode, consumers could treat it as a text stream with well-formed `<thinking>` tags and parse accordingly — no type inspection needed.