fix(completions): replay reasoning_content on thinking-mode tool calls

[Fodesu]

The bug

DeepSeek in thinking mode rejects a request with a hard 400 when it carries tools and an assistant tool-call message after the last user turn has no reasoning_content key:

400 The reasoning_content in the thinking mode must be passed back to the API

An empty string passes. A missing key or null does not. Kimi enforces the same rule.

We hit this only on continuations of persisted history (resume after a tool approval), never inside a single run. DeepSeek fills the key itself while the tool_call id is fresh, so the gap shows up once the id has expired. Two things in the provider caused the missing key:

• Response side: reasoning_content was decoded as string, so "" (DeepSeek's value for a thinking step that produced no reasoning) was indistinguishable from the key being absent, and no ReasoningPart was recorded for that step.
• Request side: reasoning_content was omitempty on a string, so an empty reasoning block was dropped from the wire.

The fix

• Both sides now use *string. A present-but-empty key yields a ReasoningPart with empty text; the stream opens one block on the first "" delta and never reopens a closed one. Any openai-chat-v1 part is sent back, "" included.
• padThinkingReplay adds an empty key to tool-call messages that carry no ReasoningPart at all (history persisted before this change) when DeepSeek or Kimi compat is on, or when the request already carries reasoning_content. Plain OpenAI and MiniMax are untouched, since OpenAI rejects unknown message fields.

Verification

• Three tests: padding conditions (table), stream markers (table), and one two-step run with an empty thinking step that checks both the recorded ReasoningPart and the replayed key.

• Live check against deepseek-v4-flash, replaying a persisted history whose tool call carries a client-generated id, so DeepSeek cannot fill the key itself. The first row reproduces the bug, the rest cover both sides of the fix:

  History	Compat	Mode	Result
  tool call without ReasoningPart	none	non-stream	400 reasoning_content ... must be passed back
  tool call with empty ReasoningPart	none	non-stream	200
  tool call with empty ReasoningPart	none	stream	200
  tool call without ReasoningPart	DeepSeek	non-stream	200, key padded

Known limitation

Through a non-official base URL with no compat option set, padding only kicks in once the conversation already carries reasoning_content. Configure WithDeepSeekChatCompletionsCompat() for proxies. A runtime fallback (retry once on the specific 400) is deferred.