fix(coderd): allow deleting chat providers used in historical chats (#24568)

Drop the `chat_model_configs.provider -> chat_providers.provider`
foreign key and soft-delete model configs when their provider is
removed. The provider row is now hard-deleted inside a transaction that
also tombstones its model configs and promotes a replacement default
when needed.

Historical chats and messages keep pointing at the soft-deleted model
config rows, which are hidden from live/admin queries but still resolve
for read. The runtime chat path already falls back to the default model
config when a soft-deleted config is looked up.

Replaces the lost FK validation in the create/update model-config
handlers with an explicit provider lookup that returns the existing
`Chat provider is not configured.` 400.

## UX

**Admin deleting a chat provider that has historical usage**

- Before: blocked with 400 `Provider models are still referenced by
existing chats.` Admins had no in-product way to remove a provider that
had ever been used.
- After: delete succeeds (204). Any model configs under that provider
are soft-deleted. If the removed provider owned the default model
config, one of the remaining live configs is auto-promoted to the new
default. The promotion is deterministic (`ensureDefaultChatModelConfig`
picks the first live config by `provider ASC, model ASC, updated_at
DESC, id DESC`); there is no picker, and no toast or response detail
names which config became the new default.

**End users with chats that used a deleted provider's model**

- Old chats still open and their history still renders unchanged.
- Sending a new turn in such a chat silently falls back to the current
default model. No banner or warning tells the user the original model is
gone.
- The model picker no longer lists the deleted model.
- If no default model config exists at all after the delete, sending a
new turn fails with `no default chat model config is available`.

**Admin creating or updating a model config against a provider that is
not configured**

- Same as before: 400 `Chat provider is not configured.` Only the
detection mechanism changed (explicit `FOR UPDATE` lookup inside the
transaction, which also serializes against a concurrent provider
delete).

**Admin updating a model config whose row disappears mid-transaction**

- Now returns the standard 404 `Resource not found or you do not have
access to this resource` instead of the previous 500 that leaked `sql:
no rows in result set` in the detail. Unrelated internal races (for
example a race on the promoted default candidate) are still reported as
500 so they are not misclassified as "your target is gone".

Closes CODAGT-23

coderd/database/queries.sql.go

@@ -4283,6 +4283,23 @@ func (q *sqlQuerier) DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID
 	return err
 }
 
+const deleteChatModelConfigsByProvider = `-- name: DeleteChatModelConfigsByProvider :exec
+UPDATE
+    chat_model_configs
+SET
+    deleted = TRUE,
+    deleted_at = NOW(),
+    updated_at = NOW()
+WHERE
+    provider = $1::text
+    AND deleted = FALSE
+`
+
+func (q *sqlQuerier) DeleteChatModelConfigsByProvider(ctx context.Context, provider string) error {
+	_, err := q.db.ExecContext(ctx, deleteChatModelConfigsByProvider, provider)
+	return err
+}
+
 const getChatModelConfigByID = `-- name: GetChatModelConfigByID :one
 SELECT
     id, provider, model, display_name, created_by, updated_by, enabled, is_default, deleted, deleted_at, created_at, updated_at, context_limit, compression_threshold, options
@@ -4699,6 +4716,37 @@ func (q *sqlQuerier) GetChatProviderByID(ctx context.Context, id uuid.UUID) (Cha
 	return i, err
 }
 
+const getChatProviderByIDForUpdate = `-- name: GetChatProviderByIDForUpdate :one
+SELECT
+    id, provider, display_name, api_key, api_key_key_id, created_by, enabled, created_at, updated_at, base_url, central_api_key_enabled, allow_user_api_key, allow_central_api_key_fallback
+FROM
+    chat_providers
+WHERE
+    id = $1::uuid
+FOR UPDATE
+`
+
+func (q *sqlQuerier) GetChatProviderByIDForUpdate(ctx context.Context, id uuid.UUID) (ChatProvider, error) {
+	row := q.db.QueryRowContext(ctx, getChatProviderByIDForUpdate, id)
+	var i ChatProvider
+	err := row.Scan(
+		&i.ID,
+		&i.Provider,
+		&i.DisplayName,
+		&i.APIKey,
+		&i.ApiKeyKeyID,
+		&i.CreatedBy,
+		&i.Enabled,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.BaseUrl,
+		&i.CentralApiKeyEnabled,
+		&i.AllowUserApiKey,
+		&i.AllowCentralApiKeyFallback,
+	)
+	return i, err
+}
+
 const getChatProviderByProvider = `-- name: GetChatProviderByProvider :one
 SELECT
     id, provider, display_name, api_key, api_key_key_id, created_by, enabled, created_at, updated_at, base_url, central_api_key_enabled, allow_user_api_key, allow_central_api_key_fallback
@@ -4729,6 +4777,37 @@ func (q *sqlQuerier) GetChatProviderByProvider(ctx context.Context, provider str
 	return i, err
 }
 
+const getChatProviderByProviderForUpdate = `-- name: GetChatProviderByProviderForUpdate :one
+SELECT
+    id, provider, display_name, api_key, api_key_key_id, created_by, enabled, created_at, updated_at, base_url, central_api_key_enabled, allow_user_api_key, allow_central_api_key_fallback
+FROM
+    chat_providers
+WHERE
+    provider = $1::text
+FOR UPDATE
+`
+
+func (q *sqlQuerier) GetChatProviderByProviderForUpdate(ctx context.Context, provider string) (ChatProvider, error) {
+	row := q.db.QueryRowContext(ctx, getChatProviderByProviderForUpdate, provider)
+	var i ChatProvider
+	err := row.Scan(
+		&i.ID,
+		&i.Provider,
+		&i.DisplayName,
+		&i.APIKey,
+		&i.ApiKeyKeyID,
+		&i.CreatedBy,
+		&i.Enabled,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.BaseUrl,
+		&i.CentralApiKeyEnabled,
+		&i.AllowUserApiKey,
+		&i.AllowCentralApiKeyFallback,
+	)
+	return i, err
+}
+
 const getChatProviders = `-- name: GetChatProviders :many
 SELECT
     id, provider, display_name, api_key, api_key_key_id, created_by, enabled, created_at, updated_at, base_url, central_api_key_enabled, allow_user_api_key, allow_central_api_key_fallback