fix(streaming): emit input_json events for server_tool_use blocks

[sean-kim05]

What

When streaming a message that contains a server-side tool call (server_tool_use, e.g. web_search), the SDK accumulates the tool's streamed input into the message snapshot but never emits an input_json stream event for it. on_input_json callbacks and code iterating the event stream therefore never see the input of server tools — only client tool_use (and, in the beta path, mcp_tool_use) fire input_json.

Root cause

accumulate_event decides what to accumulate with TRACKS_TOOL_INPUT:

# src/anthropic/lib/streaming/_messages.py
TRACKS_TOOL_INPUT = (ToolUseBlock, ServerToolUseBlock)
...
if isinstance(content, TRACKS_TOOL_INPUT):
    ...  # build up content.input from input_json_delta

But build_events decides what to emit with a narrower, hard-coded check:

elif event.delta.type == "input_json_delta":
    if content_block.type == "tool_use":          # non-beta: server_tool_use missing
        events_to_fire.append(build(InputJsonEvent, ...))

The beta path (_beta_messages.py) tracks (BetaToolUseBlock, BetaServerToolUseBlock, BetaMCPToolUseBlock) but only emits for tool_use / mcp_tool_use — again dropping server_tool_use. So the emitter and the accumulator disagree about which blocks carry input.

Fix

Emit input_json for exactly the block types whose input is accumulated, by reusing TRACKS_TOOL_INPUT via isinstance in build_events (both non-beta and beta). Emission and accumulation now share one source of truth and can't drift apart.

Tests

• New fixture tests/lib/streaming/fixtures/server_tool_use_response.txt — a stream with a web_search server_tool_use block whose input streams in as {"query": "weather in Paris"}.
• test_server_tool_use_input_json (sync + async) in both test_messages.py and test_beta_messages.py, asserting input_json events fire and the final snapshot equals the accumulated input.
• Verified RED→GREEN: without the fix, input_json_events == [] for the server_tool_use block in all four tests; with it, they fire.
• Full tests/lib/streaming/ suite (44 tests) passes; ./scripts/lint (ruff + pyright + mypy) clean.