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>`.

cli/testdata/coder_server_--help.golden

@@ -113,6 +113,12 @@ AI GATEWAY OPTIONS:
           with AI budgets. "highest" selects the group with the largest spend
           limit, and is currently the only supported value.
 
+      --ai-gateway-dump-dir string, $CODER_AI_GATEWAY_DUMP_DIR
+          Base directory for dumping AI Bridge request/response pairs to disk
+          for debugging. When set, each provider writes under a subdirectory
+          named after the provider. Sensitive headers are redacted. Leave empty
+          to disable.
+
       --ai-gateway-allow-byok bool, $CODER_AI_GATEWAY_ALLOW_BYOK (default: true)
           Allow users to provide their own LLM API keys or subscriptions. When
           disabled, only centralized key authentication is permitted.