fix(ios): serialize and finish JS license requests

[OskarEichler]

Fixes

• register pending FairPlay requests on the same main queue that emits and receives the JS callback, avoiding unsynchronized dictionary access
• queue requests that share a loadedLicenseUrl instead of overwriting the earlier request
• consume each request before processing its response so thrown responses cannot leave stale entries
• forward JS/invalid-license failures to AVContentKeyRequest.processContentKeyResponseError through the existing error path

Previously concurrent key requests for the same identifier overwrote one another. JS-reported and invalid-base64 failures also only emitted a React error; AVFoundation never received a terminal response, so the content-key request could remain pending.

Regression coverage

The v6 branch has no test suite (yarn test reports no test available), so no FairPlay regression test can be added without a new Apple DRM harness. Verified with:

• xcrun swiftc -parse ios/Video/Features/DRMManager.swift ios/Video/Features/DRMManager+OnGetLicense.swift
• YARN_NODE_LINKER=node-modules yarn build
• YARN_NODE_LINKER=node-modules yarn lint
• git diff --check

Breaking changes

None.