Follow-up to the cherry-picked PR #13897 fix. Three issues found:
1. CRITICAL: The thinking block synthesised from reasoning_content was
immediately stripped by the third-party signature management code
(Kimi is classified as _is_third_party_anthropic_endpoint). Added a
Kimi-specific carve-out that preserves unsigned thinking blocks while
still stripping Anthropic-signed blocks Kimi can't validate.
2. Empty-string reasoning_content was silently dropped because the
truthiness check ('if reasoning_content and ...') evaluates to False
for ''. Changed to 'isinstance(reasoning_content, str)' so the
tier-3 fallback from _copy_reasoning_content_for_api (which injects
'' for Kimi tool-call messages with no reasoning) actually produces
a thinking block.
3. The thinking block was appended AFTER tool_use blocks. Anthropic
protocol requires thinking -> text -> tool_use ordering. Changed to
blocks.insert(0, ...) to prepend.
04e039f687