Streamed Anthropic thinking content_block_start has no signature field #645

A streamed thinking block's content_block_start on /v1/messages is {"type":"thinking","thinking":""}, with no signature. _open_thinking() (server/anthropic_api.py) never sets it, and _event() serializes with exclude_none=True. The Anthropic SDK types ThinkingBlock.signature as a required str. The non-streaming response and the closing signature_delta already send "".

import asyncio
from freetoken.server import anthropic_api as A
from freetoken.server.generation import ReasoningDelta, ContentDelta, GenDone
async def ev():
    for e in (ReasoningDelta("x"), ContentDelta("hi"), GenDone("stop", 1, 1)):
        yield e
async def main():
    async for f in A.anthropic_event_stream(ev(), "m", 1):
        if "content_block_start" in f:
            print(f.splitlines()[1])   # ..."content_block":{"type":"thinking","thinking":""}...
asyncio.run(main())

Expected: {"type":"thinking","thinking":"","signature":""}.

Environment: main 07813248 (CPU-only check), Python 3.14.

Implementation (on my fork, with a test): e106a9e