gateway: a Chat turn's thinking is read in every spelling the client …

[TryWorld2026]

gateway: a Chat turn's thinking is read in every spelling the client sends it back in

What is wrong

A client sends the previous assistant turn's thinking back in one of three
spellings: reasoning_content (DeepSeek's, which OpenCode and most clients
send), reasoning (OpenRouter's) or reasoning_details (OpenRouter's
structured one). parseChat read only reasoning_content, so a request
translated to a non-Chat upstream lost the thinking of every turn sent another
way, and the model went on without its own reasoning. A tool-calling session
shows it from its second turn on: the assistant has a thinking block in turn
one, none in turn three.

chattidy's passthrough has read all three since mistralThinking (#494); the
parse layer never got the same reader.

What changed

The assistant branch now reads the thinking through reasoningText,
messageReasoning's own reader, in the order the passthrough keeps it:
reasoning_content, else reasoning, else reasoning_details' text. The
three fields are read as json.RawMessage so one reader handles every
spelling, as the passthrough already does.

A turn sent in a spelling the reader does not know is still carried as before:
reasoningText answers empty and no thinking part is added.

Semantic change (Gateway routing and fallback)

• Before: an assistant message whose thinking arrived as reasoning or
  reasoning_details produced a Message with no Part{Kind: Thinking}.
• After: it produces that part, so the IR carries the client's own thinking
  into whichever upstream the request is translated to.
• Reference: docs/subsystems/gateway-routing.md;
  implementation parseChat in internal/gateway/chat.go.

Verification

• TestParseChatReadsEveryReasoningSpelling fails without the change for the
  reasoning and reasoning_details spellings — the assistant turn's parts: [{Kind:tool_call ...}], the turn keeps its tool call and its thinking is gone
  — and passes with it. Its reasoning_content subtest passes on the base as
  well: upstream already reads that one, so it is a guard against dropping it
  rather than a reproducer. TestParseChatTakesTheFirstReasoningSpellingSent
  also passes on the base and stays green.
• go test -tags nogui ./internal/gateway/ for the chat, reasoning and
  thinking tests; go vet -tags nogui ./internal/gateway/...; GOOS=windows,
  GOOS=darwin and GOOS=linux builds.
• The whole internal/gateway package leaves one failure on this Windows box,
  TestSweepBridgeProjectsTakesOnlyTheBridgesFolders, which needs a symlink
  privilege Windows does not grant here and fails the same way on the untouched
  base. A second, TestPluginStreamStalledClientKeepsOtherRequestsMoving, failed
  in one run of the package and passed in another, and passes under -count=3.