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/dbmetrics/querymetrics.go

@@ -440,6 +440,14 @@ func (m queryMetricsStore) DeleteChatModelConfigByID(ctx context.Context, id uui
 	return r0
 }
 
+func (m queryMetricsStore) DeleteChatModelConfigsByProvider(ctx context.Context, provider string) error {
+	start := time.Now()
+	r0 := m.s.DeleteChatModelConfigsByProvider(ctx, provider)
+	m.queryLatencies.WithLabelValues("DeleteChatModelConfigsByProvider").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "DeleteChatModelConfigsByProvider").Inc()
+	return r0
+}
+
 func (m queryMetricsStore) DeleteChatProviderByID(ctx context.Context, id uuid.UUID) error {
 	start := time.Now()
 	r0 := m.s.DeleteChatProviderByID(ctx, id)
@@ -1344,6 +1352,14 @@ func (m queryMetricsStore) GetChatProviderByID(ctx context.Context, id uuid.UUID
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatProviderByIDForUpdate(ctx context.Context, id uuid.UUID) (database.ChatProvider, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatProviderByIDForUpdate(ctx, id)
+	m.queryLatencies.WithLabelValues("GetChatProviderByIDForUpdate").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatProviderByIDForUpdate").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatProviderByProvider(ctx context.Context, provider string) (database.ChatProvider, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatProviderByProvider(ctx, provider)
@@ -1352,6 +1368,14 @@ func (m queryMetricsStore) GetChatProviderByProvider(ctx context.Context, provid
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatProviderByProviderForUpdate(ctx context.Context, provider string) (database.ChatProvider, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatProviderByProviderForUpdate(ctx, provider)
+	m.queryLatencies.WithLabelValues("GetChatProviderByProviderForUpdate").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatProviderByProviderForUpdate").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatProviders(ctx context.Context) ([]database.ChatProvider, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatProviders(ctx)