AnthropicContentBlock / antigravity ClaudeContentItem serialized `signature`
with omitempty, so thinking blocks without an upstream signature (Chat
Completions reasoning_content, Responses reasoning without encrypted_content,
every streamed thinking content_block_start) went out with no `signature` key.
Anthropic always sends the key (`""` on content_block_start). Strict clients
such as Grok Build deserialize it as a required field and abort the response
or SSE stream with "missing field `signature`".
- apicompat: thinking MarshalJSON always emits `thinking` and `signature`
- antigravity: ClaudeContentItem MarshalJSON does the same; streamed thinking
content_block_start carries `signature: ""`
- tests: wire-shape regression tests; update TestStreamingReasoning expectation
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
