feat: register multiple Copilot providers for business and enterprise upstreams (#23811)

## Description

Adds support for multiple Copilot provider instances to route requests to different Copilot upstreams (individual, business, enterprise). Each instance has its own name and base URL, enabling per-upstream metrics, logs, circuit breakers, API dump, and routing.

## Changes

* Add Copilot business and enterprise provider names and host constants
* Register three Copilot provider instances in aibridged (default, business, enterprise)
* Update `defaultAIBridgeProvider` in `aibridgeproxy` to route new Copilot hosts to their corresponding providers

## Related

* Depends on: https://github.com/coder/aibridge/pull/240
* Closes: https://github.com/coder/aibridge/issues/152

Note: documentation changes will be added in a follow-up PR.

_Disclaimer: initially produced by Claude Opus 4.6, heavily modified and reviewed by @ssncferreira ._

coderd/aibridge/aibridge.go

@@ -20,6 +20,14 @@ const HeaderCoderToken = "X-Coder-AI-Governance-Token" //nolint:gosec // This is
 // request forwarded to aibridged for cross-service log correlation.
 const HeaderCoderRequestID = "X-Coder-AI-Governance-Request-Id"
 
+// Copilot provider.
+const (
+	ProviderCopilotBusiness   = "copilot-business"
+	HostCopilotBusiness       = "api.business.githubcopilot.com"
+	ProviderCopilotEnterprise = "copilot-enterprise"
+	HostCopilotEnterprise     = "api.enterprise.githubcopilot.com"
+)
+
 // IsBYOK reports whether the request is using BYOK mode, determined
 // by the presence of the X-Coder-AI-Governance-Token header.
 func IsBYOK(header http.Header) bool {