fix(deepseek): use non-empty reasoning_content placeholder for V4 Pro thinking mode

DeepSeek V4 Pro tightened thinking-mode validation and rejects empty-string
reasoning_content with HTTP 400:

    The reasoning content in the thinking mode must be passed back to the API.

run_agent.py injected "" at three fallback sites — the tool-call pad in
_build_assistant_message and both injection branches of
_copy_reasoning_content_for_api (cross-provider poison guard + unconditional
thinking pad). All three now emit " " (single space), which satisfies the
non-empty check on V4 Pro without leaking fabricated reasoning.

Also upgrades stale empty-string placeholders on replay: sessions persisted
before this change have reasoning_content="" pinned at creation time; when
the active provider enforces thinking-mode echo, the replay path now rewrites
"" -> " " so existing users don't 400 on their first V4 Pro turn after
updating. Non-thinking providers still round-trip "" verbatim.

Updates 9 existing assertions + adds 2 regression tests (stale-placeholder
upgrade, non-thinking verbatim preservation).

Refs #15250, #17400.
Closes #17341.

scripts/release.py

@@ -50,6 +50,7 @@ AUTHOR_MAP = {
     "rylen.anil@gmail.com": "rylena",
     "godnanijatin@gmail.com": "jatingodnani",
     "14046872+tmimmanuel@users.noreply.github.com": "tmimmanuel",
+    "657290301@qq.com": "IMHaoyan",
     "revar@users.noreply.github.com": "revaraver",
     # Matrix parity salvage batch (April 2026)
     "sr@samirusani": "samrusani",