fix(minimax): normalize unsigned thinking block starts

[rixzkiye]

Summary

• add the required empty signature field when MiniMax omits it from Anthropic thinking block-start events
• scope normalization through a provider quirk enabled only for minimax and minimax-cn
• preserve existing block signatures and later signature_delta events unchanged
• cover fragmented SSE chunks and provider isolation with regression tests

Problem

MiniMax can stream an Anthropic event shaped like:

{
  "type": "content_block_start",
  "content_block": { "type": "thinking", "thinking": "" }
}

Strict Messages clients deserialize signature on the block-start event before a later signature_delta arrives. Grok Build therefore aborts the turn with:

Internal error: "serialization error: missing field `signature`"

Anthropic-style thinking block starts use an empty signature placeholder; this PR adds only that missing envelope field. It does not synthesize or replace the provider signature.

Verification

• regression reproduced before the fix: MiniMax block-start signature was undefined
• cd tests && npx vitest run unit/minimax-thinking-signature.test.js unit/openai-responses-terminal-event.test.js unit/reasoningContentInjector.test.js unit/openai-to-claude-tools-no-type.test.js — 27 tests passed
• fragmented SSE test preserves the real signature_delta
• ESLint: 0 errors
• production Next.js build: 130/130 pages generated
• live local verification with Grok Build + minimax/MiniMax-M3: Claude-to-Claude streaming completed successfully with thinking enabled and 26 tools

Related

Companion transport/body alignment fix: #2705. No existing issue matching the client-side missing-signature serialization error was found.