fix: name map keys by value in error messages and metadata

[yu2971512385-ui]

Problem

Decoding any map that is not keyed by a string kind — including the map[any]any that gopkg.in/yaml.v2 and other decoders produce — reports every key as <interface {} Value>:

var result map[string]string
err := mapstructure.Decode(map[any]any{7: "foo"}, &result)
// '[<interface {} Value>]' expected type 'string', got unconvertible type 'int'

type S struct{ M map[int]string }

var md mapstructure.Metadata
d, _ := mapstructure.NewDecoder(&mapstructure.DecoderConfig{Result: &S{}, Metadata: &md})
d.Decode(map[string]any{"m": map[any]any{7: []string{"boom"}}})

// err:      'M[<interface {} Value>]' expected type 'string', got unconvertible type '[]string'
// md.Keys:  [M[<interface {} Value>] M[<interface {} Value>] M]

The same decode against a map[string]any source names the key correctly (M[x]), so the diagnostics silently degrade exactly for the input shape that is most common in YAML-backed configuration — the caller is told a key failed but not which one.

Root cause

decodeMapFromMap builds the name with reflect.Value.String():

for _, k := range dataVal.MapKeys() {
    fieldName := name + "[" + k.String() + "]"

reflect.Value.String() is documented as the one getter that does not panic for the wrong kind: it returns the string only for Kind() == String and "<T Value>" otherwise. For map[any]any the key values have Kind() == Interface, and for map[int]T they have Kind() == Int.

Fix

Add a small mapKeyName helper that unwraps an interface key and formats the underlying value, and use it for the field name. String keys keep taking the existing fast path, so no existing name changes:

before: 'M[<interface {} Value>]' expected type 'string', got unconvertible type '[]string'
after:  'M[7]' expected type 'string', got unconvertible type '[]string'

Tests

• TestDecodeMapKeyName — an int key that cannot be decoded into a string key must be named '[7]' in the error.
• TestMetadata_MapKeys — an any-keyed source map must contribute Vmap[foo] / Vmap[baz] to Metadata.Keys.

Both fail without the change; go test ./... passes with it.