fix(ai): scope the normalized flag, gate the Mistral stream split, correct four doc claims

Acts on a second triple-check audit.

`normalized` means one thing. The driver stamped it on both the OpenAI-shape
path and the legacy `response.normalize` path, which converts toward Anthropic
blocks — so a caller could get `normalized: true` alongside array content, and
the flag told them nothing they could branch on. The legacy branch no longer
sets it. That branch is reachable only by a direct driver call with
`response.normalize` and no `normalize`; the four wire routes pin
`normalize: false`, which skips it.

Mistral streaming, split by concern. Flattening a reasoning model's chunked
`delta.content` to a string is a correctness floor and stays ungated — the
shared handler passes the value straight to `addText`, so an array reaches the
caller as stringified objects. Splitting the thinking text out into a
`reasoning` delta is the dialect change and now sits behind the policy gate
like the non-streaming remap. On the native path the thinking text is kept
inline rather than dropped.

Mistral `finishReason` is deleted only once its value carried over. The delete
ran unconditionally, so a non-string `finishReason` with no `finish_reason`
left the choice with no finish reason at all.

Four doc claims corrected against the code paths they cover: `content` is
string-or-null on normalized responses (tool-only turns carry no text, and the
`// always a string` example comment was wrong); `reasoning_details` is not
scoped to normalized responses, since Responses models emit it either way; and
the release-date rule depends on the serving provider's own dates — OpenRouter
derives them from its live API, so models newly listed there from 2026-09-01
normalize by default.

Adds the test the copy-on-write fix was actually for: one messages array sent
through two sequential calls, asserting the caller's array is untouched and
both attempts carried the thinking signature. That is the fallback hazard; the
harness wires one provider per model, so the fallback loop itself cannot be
driven from a provider test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/docs/src/AI/chat.md

@@ -126,7 +126,7 @@ You can control this per call with the `normalize` option:
 ```js
 // Force the OpenAI format on any model, old or new:
 const response = await puter.ai.chat("Hello", { model: "claude-sonnet-5", normalize: true });
-console.log(response.message.content);   // always a string
+console.log(response.message.content);   // a string, or null on a tool-only turn
 console.log(response.finish_reason);     // "stop" | "length" | "tool_calls" | "content_filter" | vendor value
 
 // Force the vendor-native format, even on a post-cutoff model:
@@ -148,6 +148,8 @@ Normalization does not cost you the ability to continue a reasoning turn. The op
 
 One caveat. The release-date rule applies to the model that actually serves the request — if a request is rerouted to a fallback provider, the served model's release date decides.
 
+One thing to know about the release-date rule: a model's release date comes from the catalog of whichever provider serves it, and some providers report it from their own live listing. Models served through OpenRouter carry the date OpenRouter itself assigns, so a model newly listed there on or after September 1, 2026 is normalized by default without Puter shipping any change. Pin `normalize: false` if your code depends on a provider's native shape.
+
 Streaming is unaffected by normalization: streamed [`ChatResponseChunk`](/Objects/chatresponsechunk) objects already share one format across all vendors.
 
 ## Function Calling

src/docs/src/Objects/chatresponse.md

@@ -13,13 +13,13 @@ An object containing the chat message data.
 
 - `role` (String) - The role of the message sender.
 
-- `content` (String | Array) - The content of the message. A string on normalized (OpenAI-format) responses — which includes all models released on or after September 1, 2026 and any call made with `normalize: true`. On older Anthropic models without `normalize: true`, this is the vendor-native array of content blocks such as `[{ type: "text", text: "..." }]`. See [Response Normalization](/AI/chat#response-normalization).
+- `content` (String | Array) - The content of the message. On normalized (OpenAI-format) responses — which includes all models released on or after September 1, 2026 and any call made with `normalize: true` — this is a string, or `null` when the model returned only tool calls and no text. On older Anthropic models without `normalize: true`, this is the vendor-native array of content blocks such as `[{ type: "text", text: "..." }]`. See [Response Normalization](/AI/chat#response-normalization).
 
 - `tool_calls` (Array) - An optional array of [`ToolCall`](/Objects/toolcall) objects if the model wants to call tools.
 
 - `reasoning` (String) - Optional extended-thinking output, when the model exposes it. Multiple reasoning segments are joined with a blank line between them.
 
-- `reasoning_details` (Array) - Optional opaque reasoning artifacts, present on normalized responses from models that expose them: Anthropic `thinking`/`redacted_thinking` blocks with their `signature`, or OpenAI reasoning items with their `id` and `encrypted_content`. Treat the contents as opaque and resend the array verbatim to continue an extended-thinking turn — providers reject a continuation whose reasoning lost its signature. The human-readable text is in `reasoning`; this field is only for the round trip.
+- `reasoning_details` (Array) - Optional opaque reasoning artifacts from models that expose them. Present on normalized Anthropic responses, and on OpenAI Responses-API models whether or not the response was normalized. Contents: Anthropic `thinking`/`redacted_thinking` blocks with their `signature`, or OpenAI reasoning items with their `id` and `encrypted_content`. Treat the contents as opaque and resend the array verbatim to continue an extended-thinking turn — providers reject a continuation whose reasoning lost its signature. The human-readable text is in `reasoning`; this field is only for the round trip.
 
 - `tool_call_id` (String) - An optional identifier linking this message to the tool call it responds to.