[siddiqueirshad]

Summary

Fixes #5258.

When loading an application configuration from YAML (such as via fly config validate -c fly.yaml or fly deploy), stringifyYAMLMapKeys previously performed a direct type-assertion (k.(string)) when converting map[any]any keys to string. If a YAML configuration contains non-string keys (e.g. integer or boolean keys in an env: section like 1: one or true: yes), this caused flyctl to crash with:

panic: interface conversion: interface {} is int, not string

Changes

• internal/appconfig/serde.go: Use fmt.Sprint(k) instead of k.(string) when building newmap from map[any]any so that all YAML keys are consistently stringified without panic.
• internal/appconfig/serde_test.go: Added TestLoadYAMLAppConfigWithNonStringKeys to verify that YAML configurations with non-string map keys load cleanly without panic and parse into both *Config and map[string]any.