fix(streaming): fire BetaInputJsonEvent for server_tool_use blocks

[abhicris]

Closes #1421.

Bug

`lib/streaming/_beta_messages.py` fires `BetaInputJsonEvent` for `tool_use` and `mcp_tool_use` content blocks but not for `server_tool_use`, even though the API streams `input_json_delta` events for server-side tools the same way.

This was inconsistent with the accumulator's own `TRACKS_TOOL_INPUT` tuple in the same file, which does include `BetaServerToolUseBlock` — so the SDK accumulates the streamed input into `content_block.input` correctly, but never fires the parsed event to consumers. Forces consumers to bypass `BetaInputJsonEvent` and match on the raw `BetaRawContentBlockDeltaEvent` with a `BetaInputJSONDelta` delta.

Fix

Add `"server_tool_use"` to the content-block type check on line 368:

```diff
elif event.delta.type == "input_json_delta":

•    if content_block.type == "tool_use" or content_block.type == "mcp_tool_use":
  

•    if content_block.type in ("tool_use", "mcp_tool_use", "server_tool_use"):
         events_to_fire.append(
             build(
                 BetaInputJsonEvent,
                 type="input_json",
                 partial_json=event.delta.partial_json,
                 snapshot=content_block.input,
             )
         )
  

```

(Plus a comment explaining the consistency with `TRACKS_TOOL_INPUT`.)

`+8 / -1`. Affects only the parsed event surface — the accumulator's state already tracks server_tool_use correctly. Now `tool_search_tool_regex` / `web_search` / future server-side tools' input streams will surface as `BetaInputJsonEvent` to consumers using the stream helpers.

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Contributed by kcolbchain.