feat: configure multiple AI Bridge providers of the same type (#23948)

_Disclaimer: produced mostly by Claude Opus 4.6 following detailed
planning._

## Summary
- Support multiple instances of the same AI Bridge provider type via
indexed env vars (`CODER_AIBRIDGE_PROVIDER_<N>_<KEY>`), following the
`CODER_EXTERNAL_AUTH_<N>_<KEY>` pattern
- Existing single-provider env vars (`CODER_AIBRIDGE_OPENAI_KEY`, etc.)
continue to work unchanged
- Setting both a legacy env var and an indexed provider with the same
name errors at startup to prevent silent misconfiguration
- Mark legacy provider fields (`OpenAI`, `Anthropic`, `Bedrock`) as
deprecated in `AIBridgeConfig` in favor of `Providers`
  ## Example
```sh
CODER_AIBRIDGE_PROVIDER_0_TYPE=anthropic
CODER_AIBRIDGE_PROVIDER_0_NAME=anthropic-corp
CODER_AIBRIDGE_PROVIDER_0_KEY=sk-ant-corp-xxx

CODER_AIBRIDGE_PROVIDER_0_BASE_URL=https://llm-proxy.internal.example.com/anthropic

CODER_AIBRIDGE_PROVIDER_1_TYPE=anthropic
CODER_AIBRIDGE_PROVIDER_1_NAME=anthropic-direct
  CODER_AIBRIDGE_PROVIDER_1_KEY=sk-ant-direct-yyy         
  ```
  Each instance is routed by name:
- /api/v2/aibridge/**anthropic-corp**/v1/messages
- /api/v2/aibridge/**anthropic-direct**/v1/messages
Closes
[AIGOV-157](https://linear.app/codercom/issue/AIGOV-157/spike-to-understand-if-there-is-a-simple-way-to-handle-multi-api-key)

---------

Signed-off-by: Danny Kopping <danny@coder.com>

site/src/api/typesGenerated.ts

@@ -41,9 +41,23 @@ export interface AIBridgeBedrockConfig {
 // From codersdk/deployment.go
 export interface AIBridgeConfig {
 	readonly enabled: boolean;
+	/**
+	 * Deprecated: Use Providers with indexed CODER_AIBRIDGE_PROVIDER_<N>_* env vars instead.
+	 */
 	readonly openai: AIBridgeOpenAIConfig;
+	/**
+	 * Deprecated: Use Providers with indexed CODER_AIBRIDGE_PROVIDER_<N>_* env vars instead.
+	 */
 	readonly anthropic: AIBridgeAnthropicConfig;
+	/**
+	 * Deprecated: Use Providers with indexed CODER_AIBRIDGE_PROVIDER_<N>_* env vars instead.
+	 */
 	readonly bedrock: AIBridgeBedrockConfig;
+	/**
+	 * Providers holds provider instances populated from CODER_AIBRIDGE_PROVIDER_<N>_<KEY>
+	 * env vars and/or the deprecated LegacyOpenAI/LegacyAnthropic/LegacyBedrock fields above.
+	 */
+	readonly providers?: readonly AIBridgeProviderConfig[];
 	/**
 	 * Deprecated: Injected MCP in AI Bridge is deprecated and will be removed in a future release.
 	 */
@@ -109,6 +123,31 @@ export interface AIBridgeOpenAIConfig {
 	readonly key: string;
 }
 
+// From codersdk/deployment.go
+/**
+ * AIBridgeProviderConfig represents a single AI Bridge provider instance,
+ * parsed from CODER_AIBRIDGE_PROVIDER_<N>_<KEY> environment variables.
+ * This follows the same indexed pattern as ExternalAuthConfig.
+ */
+export interface AIBridgeProviderConfig {
+	/**
+	 * Type is the provider type: "openai", "anthropic", or "copilot".
+	 */
+	readonly type: string;
+	/**
+	 * Name is the unique instance identifier used for routing.
+	 * Defaults to Type if not provided.
+	 */
+	readonly name: string;
+	/**
+	 * BaseURL is the base URL of the upstream provider API.
+	 */
+	readonly base_url: string;
+	readonly bedrock_region?: string;
+	readonly bedrock_model?: string;
+	readonly bedrock_small_fast_model?: string;
+}
+
 // From codersdk/deployment.go
 export interface AIBridgeProxyConfig {
 	readonly enabled: boolean;