chore: bridged and proxied routes format is inconsistent

[pawbana]

In every provider bridged routes contain provider prefix while proxied routes don't.

Example:

• bridged routes for OpenAI provider:

  aibridge/provider/openai.go

  Lines 21 to 22 in 406f98f

  	routeChatCompletions = "/openai/v1/chat/completions" // https://platform.openai.com/docs/api-reference/chat
  	routeResponses = "/openai/v1/responses" // https://platform.openai.com/docs/api-reference/responses

• pass through routes for OpenAI provider:

  aibridge/provider/openai.go

  Lines 72 to 82 in 406f98f

  	func (p *OpenAI) PassthroughRoutes() []string {
  	return []string{
  	// See https://pkg.go.dev/net/http#hdr-Trailing_slash_redirection-ServeMux.
  	// but without non trailing slash route requests to `/v1/conversations` are going to catch all
  	"/v1/conversations",
  	"/v1/conversations/",
  	"/v1/models",
  	"/v1/models/",
  	"/v1/responses/", // Forwards other responses API endpoints, eg: https://platform.openai.com/docs/api-reference/responses/get
  	}
  	}

Format should be the same between bridged and pass through routes.

Additional pedantic not important remark that may introduce backwards compatibility issues: v1 prefix for OpenAI paths is not necessary. Docs mention setting OPENAI_BASE_URL to https://coder.example.com/api/v2/aibridge/openai/v1 which includes the v1 suffix so removing it would be problematic but there is no need for it to exist. It may cause confusion why it is there.