fix(turbosign): send required:false for optional signer fields

[nicolasiscoding]

Closes #86

Summary

TurboSign fields accept a required boolean. It defaults to true; required: false makes the field optional for the signer. This PR makes sure every SDK can send required: false and that it reaches the request body.

Go and PHP could not send it before:

• go-sdk: Field.Required was a plain bool with omitempty, so false was always dropped. It is now *bool with omitempty: nil omits the key (required), turbodocx.BoolPtr(false) sends false, using the existing BoolPtr helper.
  • Source-breaking for Go callers who wrote Required: true. Change it to Required: turbodocx.BoolPtr(true), or drop it, since omitted already means required. In-repo examples and cmd/manual are updated.
• php-sdk: Field::$required was bool $required = false and only sent when true. It is now ?bool $required = null and is sent whenever it is not null.
  • Behavior change: an explicit required: false used to be silently dropped and is now sent. On a signature or initial field, that now returns a 400 from the API instead of being ignored.

JS, Python, Java and Ruby already passed false through. Each of them gets a payload test in this PR, plus doc updates.

API rules

• Omitting required means the field is required.
• required: false on a signature or initial field is rejected with 400 OptionalNotSupported. This rule is documented on the field in each SDK.
• A non-boolean required is rejected with 400 InvalidFieldRequired.
• A recipient whose only editable fields are optional or read-only is rejected with 400 NoEditableFieldsForRecipient.

The SDKs do not validate these rules client-side. A rejection is surfaced as a 400 ValidationError.

Tests

Every SDK has a new test with three fields (required: true, required: false, and no required) that checks the serialized fields part: true, false, and the key absent. JS, Python, Go, Java and Ruby check the request captured by their mocked HTTP layer. PHP has no HTTP-mocked TurboSign test, so its test checks fields encoded the same way TurboSign builds it (json_encode of each Field::toArray()).

SDK	Change	Before fix	Command	Result
js-sdk	verified + docs	passed (pass-through)	npm run build && npx jest	336 passed
go-sdk	fixed	build failed (*bool vs bool)	go test ./..., go vet -tags manual ./cmd/manual/	ok
php-sdk	fixed	failed (required key missing)	composer test, composer phpstan, composer cs-fix -- --dry-run	369 tests OK, phpstan no errors, cs-fixer clean
java-sdk	verified + docs	passed (Gson keeps Boolean.FALSE, drops null)	mvn test -B	329 tests, 0 failures
py-sdk	verified + docs	passed (pass-through)	pytest	346 passed
ruby-sdk	verified + docs	passed (pass-through)	bundle exec rspec	330 examples, 0 failures

Note: go-sdk/examples/turbosign_advanced.go (//go:build ignore) already fails go vet on main because of an unrelated recipient.Status reference. This PR does not touch that line.

No version bump; this follows the repo convention of bumping in a separate lockstep release. Because of the Go change, that release should be a minor bump.

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