fix: deduplicate concurrent downloads in resolveModelFile

[graciegould]

Description of change

• Current behavior: Concurrent resolveModelFile calls each start separate ipull downloads to the same .ipull temp file. When the first completes and renames the file, the other instances' file handles become orphaned. On Node 22+, garbage-collected FileHandle objects are a fatal error that crashes the process.

• New behavior: A module-level Map tracks in-flight downloads by entrypoint path. Concurrent calls detect the existing download, cancel their own downloader, and await the shared promise.

• Fixes #569

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run format to apply eslint formatting
• npm run test passes with this change
• This pull request links relevant issues as Fixes #569
• There are new or updated unit tests validating the change - N/A
• Documentation has been updated to reflect this change - N/A
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[giladgd]

The issue with this approach is that the first call to resolveModelFile may pass a signal and cancel it and thus cause other downloads to be cancelled as well.
I think we can take a different approach by doing something like this:

import {acquireLock} from "lifecycle-utils";

...

const downloader = await createModelDownloader({
    ...
});

const lock = await acquireLock([resolveModelFile, "download", downloader.entrypointFilePath]);
try {
    if (await fs.pathExists(downloader.entrypointFilePath)) {
        await downloader.cancel({deleteTempFile: false});
        return downloader.entrypointFilePath;
    }


    ...

    return downloader.entrypointFilePath;
} finally {
    lock.dispose();
}

In this case if the first download gets cancelled, the next call to resolveModelFile will redownload (and use the headers, tokens and other parameters passed to that resolveModelFile call).

[graciegould]

Good call! I will update this.