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>

cli/testdata/server-config.yaml.golden

@@ -858,21 +858,11 @@ aibridgeproxy:
   # clients.
   # (default: <unset>, type: string)
   key_file: ""
-  # Comma-separated list of AI provider domains for which HTTPS traffic will be
-  # decrypted and routed through AI Bridge. Requests to other domains will be
-  # tunneled directly without decryption. Supported domains: api.anthropic.com,
-  # api.openai.com, api.individual.githubcopilot.com,
-  # api.business.githubcopilot.com, api.enterprise.githubcopilot.com, chatgpt.com.
-  # (default:
-  # api.anthropic.com,api.openai.com,api.individual.githubcopilot.com,api.business.githubcopilot.com,api.enterprise.githubcopilot.com,chatgpt.com,
-  # type: string-array)
-  domain_allowlist:
-    - api.anthropic.com
-    - api.openai.com
-    - api.individual.githubcopilot.com
-    - api.business.githubcopilot.com
-    - api.enterprise.githubcopilot.com
-    - chatgpt.com
+  # Deprecated: This value is now derived automatically from the configured AI
+  # Bridge providers' base URLs. Setting this value has no effect. This option will
+  # be removed in a future release.
+  # (default: <unset>, type: string-array)
+  domain_allowlist: []
   # URL of an upstream HTTP proxy to chain tunneled (non-allowlisted) requests
   # through. Format: http://[user:pass@]host:port or https://[user:pass@]host:port.
   # (default: <unset>, type: string)