chore: clean up env var usage in aibridge (#24783)

> AI tools where used when creating this PR

This PR removes environment variable parsing from `/aibridge` directory.

Added env variables/flags for dump dir as coder options.
Only added to new indexed provider options
(`CODER_AIBRIDGE_PROVIDER_<N>_*`) not to deprecated legacy env variables
(`CODER_AIBRIDGE_ANTHROPIC_*` and `CODER_AIBRIDGE_OPENAI_KEY_*`).

Reverted adding `MaxRetries` option as it will be removed soon due to
key failover work:
https://github.com/coder/coder/pull/24783#discussion_r3155544808

cli/server.go

@@ -2981,6 +2981,8 @@ func ReadAIBridgeProvidersFromEnv(logger slog.Logger, environ []string) ([]coder
 			provider.Key = v.Value
 		case "BASE_URL":
 			provider.BaseURL = v.Value
+		case "DUMP_DIR":
+			provider.DumpDir = v.Value
 		case "BEDROCK_BASE_URL":
 			provider.BedrockBaseURL = v.Value
 		case "BEDROCK_REGION":

cli/server_aibridge_internal_test.go

@@ -34,6 +34,7 @@ func TestReadAIBridgeProvidersFromEnv(t *testing.T) {
 				"CODER_AIBRIDGE_PROVIDER_0_NAME=anthropic-zdr",
 				"CODER_AIBRIDGE_PROVIDER_0_KEY=sk-ant-xxx",
 				"CODER_AIBRIDGE_PROVIDER_0_BASE_URL=https://api.anthropic.com/",
+				"CODER_AIBRIDGE_PROVIDER_0_DUMP_DIR=/tmp/aibridge-dump",
 			},
 			expected: []codersdk.AIBridgeProviderConfig{
 				{
@@ -41,6 +42,7 @@ func TestReadAIBridgeProvidersFromEnv(t *testing.T) {
 					Name:    "anthropic-zdr",
 					Key:     "sk-ant-xxx",
 					BaseURL: "https://api.anthropic.com/",
+					DumpDir: "/tmp/aibridge-dump",
 				},
 			},
 		},