fix: case-insensitive key match for structs with more than 16 fields (#427)

[chiliec]

What

Fixes #427.

Structs with more than 16 fields didn't get their values populated when a JSON key differed in case from the Go field name (e.g. someField1 for SomeField1). Structs with 16 or fewer fields worked fine, and encoding/json matches case-insensitively in both cases, so the two decode paths disagreed.

Fix

The bitmap-optimized key decoder (used for ≤16 fields) already matches keys case-insensitively. The non-optimized fallback path (decodeKey / decodeKeyStream, taken for >16 fields) only did an exact fieldMap[k] lookup. fieldMap is already populated with lowercased key aliases at compile time, so falling back to fieldMap[strings.ToLower(k)] on a miss restores parity with the optimized path and with encoding/json.

Tests

Added TestIssue427 in decode_test.go covering both Unmarshal and the streaming Decoder.

Validation

$ go test -run 'TestIssue427' -v .
--- PASS: TestIssue427 (0.00s)

RED→GREEN (reverting only the source change):

$ git stash push internal/decoder/struct.go && go test -run TestIssue427 .
FAIL    github.com/goccy/go-json
$ git stash pop && go test -run TestIssue427 .
ok      github.com/goccy/go-json

Full suite green, gofmt clean:

$ go test ./...
ok  github.com/goccy/go-json
ok  github.com/goccy/go-json/internal/decoder
ok  github.com/goccy/go-json/internal/encoder
ok  github.com/goccy/go-json/test/cover
ok  github.com/goccy/go-json/test/example

Happy to adjust.