Fix panic when a YAML config has a non-string map key

[RaphaelFakhri]

Change Summary

What and Why: stringifyYAMLMapKeys converts the map[any]any values that the YAML decoder returns for maps with non-string keys into map[string]any. It asserts each key is a string, so a config such as env: {1: one} or a true: key panics with interface conversion: interface {} is int, not string in LoadConfig and LoadConfigAsMap.

How: Format the key with fmt.Sprint instead of asserting its type, so 1 becomes "1", which matches how TOML and JSON configs treat keys. Adds a test that loads and strictly maps a YAML config with an integer key and a boolean key.

Related to: none

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Documentation

• Fresh Produce
• In superfly/docs, or asked for help from docs team
• n/a

Fixes #5258