Validate each token of multi-value enum fields

[vsaraikin]

Fixes #754.

Fields of type MULTIPLEVALUESTRING / MULTIPLESTRINGVALUE / MULTIPLECHARVALUE carry a space-delimited set of enum tokens (per the FIX spec, e.g. ExecInst = "1 5" means NOTHELD + HELD). validateField looked the entire raw value up in the enum map:

if _, validValue := allowedValues[string(field.value)]; !validValue {
    return ValueIsIncorrect(field.tag)
}

so any legal combination of two or more values was rejected with ValueIsIncorrect ("out of range"), even though each token is individually valid. Single-value fields still passed, which masked the bug.

Fix

For the three multi-value field types, split the value on ' ' and validate every component against the enum map; all other types keep the original single-lookup path.

Testing

Added two cases to TestValidate (FIX.4.3, ExecInst tag 18, a MULTIPLEVALUESTRING):

• "Y M" (two valid ExecInst enums) → accepted. Fails on the current code (Unexpected reject: Value is incorrect), passes with the fix.
• "Y ZZ" (one valid, one invalid token) → still correctly rejected with ValueIsIncorrect and the right ref tag.

The full package suite passes with no regression; gofmt clean.

Note: the code generator (cmd/generate-fix/internal/template_helpers.go) has the analogous single-lookup pattern in generated per-field setters — happy to follow up on that separately if you'd like it addressed too.