refactor!: remove default organization model routes (#28632) - #28704

> [!IMPORTANT]
> Most of the added lines in this PR are generated API reference content
for publishing the organization-scoped `GET` and `POST
/api/v2/organizations/{organization}/chats/models` replacements. There
is no matching generated-doc deletion because the three removed
default-organization endpoints are experimental and are not present in
the generated public API reference on the current base. The removed
paths remain only as explicit 404 reservations, with no
default-organization shim or functional handler.

## Summary

Remove the unused default-organization chat model collection routes:

```text
GET  /api/experimental/chats/models
GET  /api/experimental/chats/model-configs
POST /api/experimental/chats/model-configs
```

Use the organization-scoped collection instead:

```text
GET  /api/v2/organizations/{organization}/chats/models
POST /api/v2/organizations/{organization}/chats/models
```

## Context

The intention of PR #28440 was to consolidate model availability into
the organization-scoped models collection for CODAGT-898 and remove the
superseded collection routes. That PR removed
`/organizations/{organization}/chats/models/available`, but it missed
these older default-organization routes and explicitly retained one of
them. This PR completes the intended #28440 cutover. The repository has
no SDK, CLI, or frontend consumer for the removed routes.

This change also prevents PR #28496 from promoting the missed
default-organization `/chats/models` route into the stable `/api/v2`
API.

This change is separate from the one-release `/api/experimental`
compatibility window in CODAGT-921. That window preserves experimental
versions of the intended stable API. It does not require compatibility
routes for unused deployment-scoped endpoints.

Refs CODAGT-898.

## Breaking change

Clients that call the removed routes must use the organization-scoped
model collection and provide an organization.

## Changelog

Remove unused default-organization Coder Agents model API routes. API
clients must use the organization-scoped chat models collection.

> [!NOTE]
> Coder Agents generated this pull request on Ethan Dickson's behalf.

(cherry picked from commit 0f5c7b3)