fix: allow changing model when editing earlier chat message (#25084)

Editing a previous user message and selecting a different model in the
picker silently kept using the original model: the selection was dropped
on the frontend, in the SDK, and in the backend, so both the replacement
user message and the assistant turn that followed ran against the old
model.

Plumb the selected model through all three layers (`AgentChatPage`,
`codersdk.EditChatMessageRequest`, `chatd.EditMessageOptions` /
`Server.EditMessage`), defaulting to the original message's model when
the client does not specify one. The existing `InsertChatMessages` CTE
already advances `chats.last_model_config_id` when the inserted
message's model differs, so the assistant turn picks up the new
selection without further changes. The new model is validated inside the
transaction, so an unknown ID rolls the edit back and returns a 400
`Invalid model config ID.`, mirroring the `SendMessage` path.

Refs: CODAGT-345

This change was generated by a Coder agent.

<details>
<summary>Implementation plan</summary>

# CODAGT-345: Editing an earlier message cannot change model

## Problem

When editing a previous user message in a chat, the user can change the
model in the model picker, but the backend keeps using the original
message's model. The model selection is dropped at three layers:

1. **Frontend:** `AgentChatPage.tsx`'s edit branch builds an
`EditChatMessageRequest` that omits `model_config_id`. The new-message
branch (a few lines below) does include it.
2. **SDK:** `codersdk.EditChatMessageRequest` has no `ModelConfigID`
field at all.
3. **Backend:** `chatd.EditMessageOptions` has no model field, and
`Server.EditMessage` always copies the original message's
`ModelConfigID` into the replacement message.

Once the replacement user message is inserted with the original model,
the `InsertChatMessages` CTE leaves `chats.last_model_config_id`
unchanged, so the assistant turn that follows runs against the old
model.

## Fix

Plumb the selected model through all three layers, defaulting to the
original message's model when the client doesn't override it. This
mirrors the `SendMessage` path, which already accepts a
`model_config_id` and validates it via
`resolveSendMessageModelConfigID`.

### Backend

- `codersdk/chats.go`: add `ModelConfigID *uuid.UUID` to
`EditChatMessageRequest`.
- `coderd/x/chatd/chatd.go`:
  - Add `ModelConfigID uuid.UUID` to `EditMessageOptions`.
- In `EditMessage`, after fetching the edited message, resolve the
model: if `opts.ModelConfigID != uuid.Nil`, validate it exists with
`tx.GetChatModelConfigByID` (using `chatdModelConfigLookupContext`),
otherwise keep `editedMsg.ModelConfigID.UUID`. Pass the resolved ID into
`newChatMessage(...)`.
  - Reuse the existing `ErrInvalidModelConfigID` sentinel.
- `coderd/exp_chats.go` (`patchChatMessage`):
- Read `req.ModelConfigID` (nil-safe), pass into
`chatd.EditMessageOptions`.
- Add a `case xerrors.Is(editErr, chatd.ErrInvalidModelConfigID)` arm
returning 400 `Invalid model config ID.`, matching the
`postChatMessages` handler.

### Frontend

- `site/src/pages/AgentsPage/AgentChatPage.tsx`:
- In the edit branch, set `model_config_id: effectiveSelectedModel ||
undefined` on the `EditChatMessageRequest`.
- On success, persist the chosen model to `lastModelConfigIDStorageKey`
so the next chat from this browser keeps the same default. Mirrors the
new-message branch.

### Generated

- `make site/src/api/typesGenerated.ts` and `make
coderd/apidoc/swagger.json` produce the updated `EditChatMessageRequest`
schema in `typesGenerated.ts`, `coderd/apidoc/{docs.go,swagger.json}`,
and `docs/reference/api/{chats.md,schemas.md}`.

## Tests

- `coderd/x/chatd/chatd_test.go`:
- `TestEditMessageWithModelConfigOverride`: edit with a different model
-> replacement message and `chats.LastModelConfigID` use the new model.
- `TestEditMessagePreservesModelConfigByDefault`: edit without
`ModelConfigID` -> original model preserved.
- `TestEditMessageRejectsUnknownModelConfig`: passes a random UUID ->
`ErrInvalidModelConfigID`, original message still present,
`LastModelConfigID` unchanged (rollback).
- `coderd/exp_chats_test.go` (under `TestPatchChatMessage`):
- `ChangesModel`: end-to-end via SDK; `edited.Message.ModelConfigID` and
`chat.LastModelConfigID` both match the new model.
- `InvalidModelConfigID`: random UUID -> 400 `Invalid model config ID.`.

</details>

docs/reference/api/chats.md

@@ -2013,7 +2013,8 @@ Experimental: this endpoint is subject to change.
       "text": "string",
       "type": "text"
     }
-  ]
+  ],
+  "model_config_id": "f5fb4d91-62ca-4377-9ee6-5d43ba00d205"
 }
 ```
 

docs/reference/api/schemas.md

@@ -6598,15 +6598,17 @@ CreateWorkspaceRequest provides options for creating a new workspace. Only one o
       "text": "string",
       "type": "text"
     }
-  ]
+  ],
+  "model_config_id": "f5fb4d91-62ca-4377-9ee6-5d43ba00d205"
 }
 ```
 
 ### Properties
 
-| Name      | Type                                                      | Required | Restrictions | Description |
-|-----------|-----------------------------------------------------------|----------|--------------|-------------|
-| `content` | array of [codersdk.ChatInputPart](#codersdkchatinputpart) | false    |              |             |
+| Name              | Type                                                      | Required | Restrictions | Description                                                                                                                                                                  |
+|-------------------|-----------------------------------------------------------|----------|--------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `content`         | array of [codersdk.ChatInputPart](#codersdkchatinputpart) | false    |              |                                                                                                                                                                              |
+| `model_config_id` | string                                                    | false    |              | Model config ID when set, overrides the model used for the replacement user message and the assistant turn that follows. When nil the original message's model is preserved. |
 
 ## codersdk.EditChatMessageResponse