fix(streaming): wrap mid-stream transport errors as APIError subclasses

[intertermux-code]

Fixes #1919.

The initial send wraps transport failures (httpx2.TimeoutException → APITimeoutError, other transport errors → APIConnectionError), but once the response was streaming, _streaming.py iterated it with no handling — a read timeout or dropped connection mid-stream surfaced as the raw httpx exception, outside the APIError hierarchy and outside max_retries.

Stream._iter_events and AsyncStream._iter_events now apply the same translation as the initial send, with the same passthrough for SDK-originated errors. Added sync/async regression tests using a mock transport that dies mid-stream.

[sigley]

Two things look worth resolving before this lands.

First, this is the same open issue already covered by #1920 and #1922. #1919 itself points to both of them, and both are still open. #1920 in particular changes these same two _iter_events methods and already scopes the non-timeout handler to httpx2.TransportError. Unless this PR intentionally differs, it would be better to coordinate on one branch rather than leave a third competing fix for the same two hunks.

Second, this head is broader than those earlier fixes:

except Exception as err:
    if isinstance(err, AnthropicError):
        raise
    raise APIConnectionError(...)

That try covers both response.iter_bytes() and the SSE byte decoder. With the default decoder, for example, an invalid UTF-8 SSE line raises UnicodeDecodeError at raw_line.decode("utf-8"); on this branch that becomes APIConnectionError, even though it is not an httpx2 transport failure. A custom SSEBytesDecoder raising its own ordinary exception is relabeled the same way. #1920/#1922 deliberately use except httpx2.TransportError so decoder/parsing failures keep their original identity.

Could the second handler be narrowed to httpx2.TransportError and a non-transport decoder-error regression be added? That would restore the exception hierarchy for connection failures without turning decoder bugs or malformed stream data into network errors.

[intertermux-code]

Done — narrowed both _iter_events handlers to except httpx2.TransportError, which brings this PR's scope exactly in line with #1920. You were right about the broad except Exception: it also covered the SSE decoder, so a UnicodeDecodeError from a malformed SSE line (or an exception from a custom SSEBytesDecoder) would have been relabeled as APIConnectionError.

Also added test_mid_stream_decoder_error_keeps_original_identity (sync + async) asserting that decoder errors keep their original identity — it fails against the old handler and passes now. The transport-error wrapping tests still pass unchanged.

On the duplication point: noted. #1920 has been untouched for a month, so I'll keep this branch live as the active one — happy to consolidate onto #1920 instead if you'd rather.