[Bug]: Each transfer leaks a URLSession — bulk downloads exhaust file descriptors

### Current Behavior

IONFLTRManager.downloadFile() / uploadFile() create a new URLSession per transfer:

```swift
let session = URLSession(configuration: .default, delegate: delegate, delegateQueue: nil)
session.downloadTask(with: request).resume()
```

The session is never invalidated. Per Apple's documentation, a URLSession strongly retains its delegate and its resources (including the keep-alive socket) until finishTasksAndInvalidate() / invalidateAndCancel() is called — otherwise it leaks until app exit. So every completed transfer leaks one session, one delegate, and one open socket / file descriptor.

### Expected Behavior

Invalidate the session in the final delegate callback, in both IONFLTRDownloadDelegate and IONFLTRUploadDelegate:

```swift
func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: (any Error)?) {
defer { session.finishTasksAndInvalidate() }
// existing handling …
}
```

### Reproduction

.

### Screenshots / Media

_No response_

### Environment Details

```Shell
@capacitor/cli: 8.2.0
@capacitor/core: 8.2.0
@capacitor/android: 8.2.0
@capacitor/ios: 8.2.0

npm: 11.17.0
node: 26.4.0
pod: 1.16.2
```

### Versions Affected

1.0.3

### Platforms Affected

- [x] iOS
- [ ] Android
- [ ] Web

### Notes / Comments

_No response_

Start by locating IONFLTRManager.downloadFile() and uploadFile(), then inspect the final URLSession delegate callbacks in IONFLTRDownloadDelegate and IONFLTRUploadDelegate. Confirm that both transfer paths invalidate their sessions after completion and that completed bulk transfers no longer retain the session resources.

Written by the indexing model from the issue text.