fix(ios): validate signingUrl and emit the documented error codes

[IronTony]

Brings the iOS module up to the contract the README has always described and the Android module now honours. Both changes are breaking, so they belong in 2.0.0 or they wait for 3.0.0.

Closes #5.

Validate signingUrl (#5)

DSMEnvelopesManager.presentCaptiveSigning(withPresenting:signingUrl:...) validates nothing and presents unconditionally. A blank or non-https URL rendered an empty signing controller whose completion never fired, so the JS promise never settled. CaptiveSigningUrlRecord defaults signingUrl to "", which means an omitted field reached the SDK without a malformed URL being involved at all.

The guard sits after the isInitialized check and before the stateQueue.sync that assigns pendingCompletion, so a rejected URL never claims the slot and a later valid call is not refused as "already in progress". Same exception type and message string as the Android guard, so the two platforms reject identically.

Emit the documented error codes

Expo derives a code from the exception class name when none is set, so NotInitializedException reached JS as ERR_NOT_INITIALIZED rather than the not_initialized the error table has listed since 1.0.0. Android got explicit codes in the previous release, which left the platforms disagreeing on every single code. All five exceptions now set theirs.

Separately, both .failure branches in DocuSignModule.swift hard-coded "signing_failed", which flattened presentation_failed and hid which stage had failed. They now forward the failure's own code. promise.reject(error) is not the alternative: it wraps anything that is not an Exception, so a raw SDK NSError would surface as ERR_UNEXPECTED.

Migration

Before (iOS)	After (both platforms)
ERR_NOT_INITIALIZED	not_initialized
ERR_NOT_LOGGED_IN	not_logged_in
ERR_LOGIN_FAILED	login_failed
ERR_SIGNING_FAILED	signing_failed
ERR_PRESENTATION_FAILED (never reached, flattened to signing_failed)	presentation_failed

Rejection messages on the failure path are now the underlying error text on its own, where they previously carried a DocuSign signing failed: prefix.

Verification

swiftc -parse clean on all three Swift files. npm run build, npm run lint and 18/18 Jest tests pass.

Worth stating plainly: CI compiles no Swift and runs no iOS tests, so nothing in this repo can catch a type error in these files. The native changes are verified by parsing and by reading against the Android implementation they mirror. A device run against a real envelope before tagging 2.0.0 would be the honest bar.

Not included

The .failure double-emit that Android had does not exist here. The manager's SDK-error path calls resolvePending(.success(outcome)) with status: "error", so the module's failure branch never runs for signing errors. Different from Android, internally consistent, left alone.