refactor: load AI providers from the database at startup (#25672)

Replace the env-based `BuildProviders` with a DB-backed loader. The database is now the single source of truth for runtime provider configuration; env config arrives via `SeedAIProvidersFromEnv` (run at boot) and `BuildProviders` reads it back as `aibridge.Provider` instances. `cli/server.go` and `enterprise/cli/server.go` both call the same path, so aibridged and aibridgeproxyd see the same provider set.

Per-provider `DumpDir` is replaced by a top-level `CODER_AI_GATEWAY_DUMP_DIR` base; each provider's effective dump path is `<base>/<provider name>`.

docs/reference/api/general.md

@@ -185,6 +185,7 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
           "base_url": "string",
           "key": "string"
         },
+        "api_dump_dir": "string",
         "bedrock": {
           "access_key": "string",
           "access_key_secret": "string",
@@ -213,7 +214,6 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
             "bedrock_model": "string",
             "bedrock_region": "string",
             "bedrock_small_fast_model": "string",
-            "dump_dir": "string",
             "name": "string",
             "type": "string"
           }