[backlog] Minimax thinking concatenated into content (upstream #511)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #511
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: partial · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    Think-tag extraction `transform/shared` `ExtractInlineThinkTags` / reasoning_content paths exist; Minimax-specific regression not isolated — thinking may be concatenated into content.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Minimax thinking/reasoning separated from assistant content per product contract

• Inline think-tag extraction covers Minimax variants

• Stream and non-stream fixtures for Minimax thinking

• Client-visible content excludes raw think tags when extracted

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.