feat(config): add support for disabling OpenAI compatibility providers

- Introduced a `Disabled` flag to OpenAI compatibility configurations.
- Updated routing, auth selection, and API handling logic to respect the `Disabled` state.
- Extended relevant APIs, YAML configurations, and data structures to include the `Disabled` field.
- Adjusted all relevant loops and filters to skip disabled providers.

Closes: #3060 #3059 #2977

internal/watcher/diff/openai_compat.go

@@ -66,6 +66,9 @@ func describeOpenAICompatibilityUpdate(oldEntry, newEntry config.OpenAICompatibi
 	oldModelCount := countOpenAIModels(oldEntry.Models)
 	newModelCount := countOpenAIModels(newEntry.Models)
 	details := make([]string, 0, 3)
+	if oldEntry.Disabled != newEntry.Disabled {
+		details = append(details, fmt.Sprintf("disabled %t -> %t", oldEntry.Disabled, newEntry.Disabled))
+	}
 	if oldKeyCount != newKeyCount {
 		details = append(details, fmt.Sprintf("api-keys %d -> %d", oldKeyCount, newKeyCount))
 	}