refactor: drop chat_model_configs provider column (#26877)

The provider type already lives authoritatively in ai_providers.type,
reachable on every active row through ai_provider_id, which the
chat_model_configs_ai_provider_required_when_active CHECK makes
mandatory. The stored provider string was a denormalized copy the system
kept in sync with a startup backfill and no longer needs.

Every surface now derives provider type from the linked ai_providers
row. Telemetry is the one exception: it keeps emitting provider, now
sourced from ai_providers.type via a JOIN, so the BigQuery column and the
Nexus dashboards that read it are unaffected. The experimental HTTP/SDK
response drops provider and makes ai_provider_id required, since those
endpoints return only active configs; consumers resolve provider type
from ai_provider_id and the AI providers listing.

This ships in a single release with no compatibility window: production
reads the table via SELECT *, so a pre-drop binary fails config reads the
moment the column is gone. Operators must scale to zero before upgrading,
and there is no rollback.

Closes CODAGT-599

coderd/database/migrations/testdata/fixtures/000475_chat_model_config_soft_deleted.up.sql

@@ -0,0 +1,41 @@
+-- Soft-deleted chat model config whose provider never had an ai_providers
+-- backfill match, so it reaches later migrations with ai_provider_id IS NULL.
+--
+-- This row exercises the 000534 down migration's
+-- `UPDATE ... SET provider = '' WHERE provider IS NULL` sweep: its NULL
+-- ai_provider_id means the backfill join leaves provider NULL, and the sweep
+-- must populate it before `ALTER COLUMN provider SET NOT NULL`.
+--
+-- It is inserted at version 000475 (after 000474 dropped the provider foreign
+-- key) so the provider value need not reference a chat_providers row, and the
+-- 000504/000505 backfill (which matches on `cmc.provider = cp.provider`) skips
+-- it. `deleted = TRUE` keeps it out of idx_chat_model_configs_single_default
+-- and satisfies chat_model_configs_ai_provider_required_when_active (added in
+-- 000505), which permits a NULL ai_provider_id only for deleted rows.
+INSERT INTO chat_model_configs (
+    id,
+    provider,
+    model,
+    display_name,
+    enabled,
+    is_default,
+    deleted,
+    deleted_at,
+    context_limit,
+    compression_threshold,
+    created_at,
+    updated_at
+) VALUES (
+    'b3a1d2c4-5e6f-4a7b-8c9d-0e1f2a3b4c5d',
+    'legacy-removed',
+    'legacy-model',
+    'Legacy Soft Deleted',
+    FALSE,
+    FALSE,
+    TRUE,
+    '2024-01-01 00:00:00+00',
+    200000,
+    70,
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00'
+);