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>

coderd/apidoc/swagger.json

@@ -11651,10 +11651,20 @@
 			"type": "object",
 			"properties": {
 				"anthropic": {
-					"$ref": "#/definitions/codersdk.AIBridgeAnthropicConfig"
+					"description": "Deprecated: Use Providers with indexed CODER_AIBRIDGE_PROVIDER_\u003cN\u003e_* env vars instead.",
+					"allOf": [
+						{
+							"$ref": "#/definitions/codersdk.AIBridgeAnthropicConfig"
+						}
+					]
 				},
 				"bedrock": {
-					"$ref": "#/definitions/codersdk.AIBridgeBedrockConfig"
+					"description": "Deprecated: Use Providers with indexed CODER_AIBRIDGE_PROVIDER_\u003cN\u003e_* env vars instead.",
+					"allOf": [
+						{
+							"$ref": "#/definitions/codersdk.AIBridgeBedrockConfig"
+						}
+					]
 				},
 				"circuit_breaker_enabled": {
 					"description": "Circuit breaker protects against cascading failures from upstream AI\nprovider rate limits (429, 503, 529 overloaded).",
@@ -11683,7 +11693,19 @@
 					"type": "integer"
 				},
 				"openai": {
-					"$ref": "#/definitions/codersdk.AIBridgeOpenAIConfig"
+					"description": "Deprecated: Use Providers with indexed CODER_AIBRIDGE_PROVIDER_\u003cN\u003e_* env vars instead.",
+					"allOf": [
+						{
+							"$ref": "#/definitions/codersdk.AIBridgeOpenAIConfig"
+						}
+					]
+				},
+				"providers": {
+					"description": "Providers holds provider instances populated from CODER_AIBRIDGE_PROVIDER_\u003cN\u003e_\u003cKEY\u003e\nenv vars and/or the deprecated LegacyOpenAI/LegacyAnthropic/LegacyBedrock fields above.",
+					"type": "array",
+					"items": {
+						"$ref": "#/definitions/codersdk.AIBridgeProviderConfig"
+					}
 				},
 				"rate_limit": {
 					"type": "integer"
@@ -11803,6 +11825,32 @@
 				}
 			}
 		},
+		"codersdk.AIBridgeProviderConfig": {
+			"type": "object",
+			"properties": {
+				"base_url": {
+					"description": "BaseURL is the base URL of the upstream provider API.",
+					"type": "string"
+				},
+				"bedrock_model": {
+					"type": "string"
+				},
+				"bedrock_region": {
+					"type": "string"
+				},
+				"bedrock_small_fast_model": {
+					"type": "string"
+				},
+				"name": {
+					"description": "Name is the unique instance identifier used for routing.\nDefaults to Type if not provided.",
+					"type": "string"
+				},
+				"type": {
+					"description": "Type is the provider type: \"openai\", \"anthropic\", or \"copilot\".",
+					"type": "string"
+				}
+			}
+		},
 		"codersdk.AIBridgeProxyConfig": {
 			"type": "object",
 			"properties": {