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/querier.go

@@ -113,6 +113,7 @@ type sqlcQuerier interface {
 	// archive-cleanup retry).
 	DeleteChatDebugDataByChatID(ctx context.Context, arg DeleteChatDebugDataByChatIDParams) (int64, error)
 	DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) error
+	DeleteChatModelConfigsByProvider(ctx context.Context, provider string) error
 	DeleteChatProviderByID(ctx context.Context, id uuid.UUID) error
 	DeleteChatQueuedMessage(ctx context.Context, arg DeleteChatQueuedMessageParams) error
 	DeleteChatUsageLimitGroupOverride(ctx context.Context, groupID uuid.UUID) error
@@ -327,7 +328,9 @@ type sqlcQuerier interface {
 	GetChatModelConfigsForTelemetry(ctx context.Context) ([]GetChatModelConfigsForTelemetryRow, error)
 	GetChatPlanModeInstructions(ctx context.Context) (string, error)
 	GetChatProviderByID(ctx context.Context, id uuid.UUID) (ChatProvider, error)
+	GetChatProviderByIDForUpdate(ctx context.Context, id uuid.UUID) (ChatProvider, error)
 	GetChatProviderByProvider(ctx context.Context, provider string) (ChatProvider, error)
+	GetChatProviderByProviderForUpdate(ctx context.Context, provider string) (ChatProvider, error)
 	GetChatProviders(ctx context.Context) ([]ChatProvider, error)
 	GetChatQueuedMessages(ctx context.Context, chatID uuid.UUID) ([]ChatQueuedMessage, error)
 	// Returns the chat retention period in days. Chats archived longer