fix(ai): gate the Mistral remap, unify the reasoning join, stop mutating caller messages

Acts on a triple-check audit of this branch.

Gate the Mistral dialect remap. The camelCase→snake_case rewrite and the
chunked-content flattening were firing for every Mistral call regardless of
`normalize` or the cutoff, deleting `finishReason` and `message.toolCalls` out
from under any caller reading them. Both now sit behind the policy resolution
the driver already used, extracted as `shouldPresentAsOpenAI` so the provider
and the driver cannot drift. The streaming chunk-array split stays ungated:
handing an array to `addText` is a plain bug, and streamed chunks are
provider-uniform by design. The conformance matrix now passes `normalize: true`,
which is the contract it was always testing.

Unify the reasoning join. Three code paths produced two separators while one
doc sentence described them all: the coercer joined thinking segments with '',
the Responses handler and Mistral with '\n\n'. The coercer now matches, and
chatresponse.md's claim is true for every path it covers. Text blocks still
join with '' — Anthropic splits prose mid-sentence across them.

finish_reason is an open set. chat.md's normalize bullet and the SDK
ChatMessage typedef still declared a closed four-value set, contradicting the
documented pass-through of unmapped vendor reasons and the coercer that
implements it.

Stop mutating caller messages. Both reasoning-replay input paths deleted
output-only fields from the caller's own message objects, which the driver
reuses across fallback attempts. Both strip a copy now; tests pass a frozen
message through each.

Drop three dead things the type cleanup left: the no-op ChatProvider
checkModeration stub (no subclasses, no callers — its removal restores a
pre-existing baselined TS2420), the redundant second normalizeReasoningContent
call in BytePlus and ZAI, and the coercer's bare-string branch that no provider
reaches. A bare string now passes through by reference instead of being
coerced.

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

src/docs/src/AI/chat.md

@@ -35,7 +35,7 @@ An object containing the following properties:
 - `tools` (Array) (Optional) - Function definitions the AI can call. See [Function Calling](#function-calling) for details.
 - `reasoning_effort` / `reasoning.effort` (String) (Optional) - Controls how much effort reasoning models spend thinking. Supported values: `none`, `minimal`, `low`, `medium`, `high`, and `xhigh`. Lower values give faster responses with less reasoning. OpenAI models and Meta's Muse Spark models only; Muse Spark always reasons, so `none` is ignored for it.
 - `verbosity` / `text.verbosity` (String) (Optional) - Controls how long or short responses are. Supported values: `low`, `medium`, and `high`. Lower values give shorter responses. OpenAI models only.
-- `normalize` (Boolean) (Optional) - Controls the format of the non-streaming response. When `true`, the response is normalized to the OpenAI format regardless of the model's vendor: `message.content` is a string, tool calls appear as `message.tool_calls`, and `finish_reason` is one of `stop`, `length`, `tool_calls`, or `content_filter`. When `false`, the response keeps the vendor's native format (for Anthropic models, an array of content blocks). When unset, `puter.ai.normalize` applies if you assigned it; otherwise **models released on or after September 1, 2026 return normalized (OpenAI-format) responses by default**, and older models keep their current behavior. Streaming responses are unaffected — chunks already share one format across vendors. See [Response normalization](#response-normalization).
+- `normalize` (Boolean) (Optional) - Controls the format of the non-streaming response. When `true`, the response is normalized to the OpenAI format regardless of the model's vendor: `message.content` is a string, tool calls appear as `message.tool_calls`, and `finish_reason` is one of `stop`, `length`, `tool_calls`, or `content_filter` — or the vendor's own stop reason, passed through unchanged when it has no OpenAI equivalent. When `false`, the response keeps the vendor's native format (for Anthropic models, an array of content blocks). When unset, `puter.ai.normalize` applies if you assigned it; otherwise **models released on or after September 1, 2026 return normalized (OpenAI-format) responses by default**, and older models keep their current behavior. Streaming responses are unaffected — chunks already share one format across vendors. See [Response normalization](#response-normalization).
 - `compaction` (Boolean | Object) (Optional) - Opt into inline context compaction for long conversations. Pass `true` to enable it with provider defaults, or `{ trigger_tokens: number }` to set the token threshold at which earlier context is summarized. When the model compacts, you receive a `compaction` chunk while streaming (or a `compaction` field on the result when not streaming) containing an opaque `encrypted_content` summary. Resend that item in `messages` on the next turn in place of the summarized history. The compaction chunk shape is identical across providers, so the same code works whether `model` is an OpenAI or Anthropic model. See [Compaction](#compaction).
 
 #### `testMode` (Boolean) (Optional)
@@ -127,7 +127,7 @@ You can control this per call with the `normalize` option:
 // 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.finish_reason);     // "stop" | "length" | "tool_calls" | "content_filter"
+console.log(response.finish_reason);     // "stop" | "length" | "tool_calls" | "content_filter" | vendor value
 
 // Force the vendor-native format, even on a post-cutoff model:
 const native = await puter.ai.chat("Hello", { model: "claude-sonnet-5", normalize: false });