Preserve blob content-type on save in BlobContainerFS

[Copilot]

Saving a blob file via VS Code overwrote the existing content-type with a value derived from the file extension (e.g. application/javascript; charset=UTF-8 → application/javascript), discarding charset and other parameters.

Changes

• src/BlobContainerFS.ts — writeFile: Replaced the separate blobClient.exists() call with blobClient.getProperties(), which both checks existence (via 404 catch) and retrieves the current content-type in a single round-trip. The existing content-type is then forwarded on upload; mime.getType() is only used as a fallback for new blobs.

// Before: always derived content-type from file extension
await blobClient.getBlockBlobClient().uploadData(content, {
    blobHTTPHeaders: {
        blobContentType: mime.getType(uri.path) || undefined
    }
});

// After: preserve existing content-type; fall back to mime detection for new blobs
let existingProperties: BlobGetPropertiesResponse | undefined;
try {
    existingProperties = await blobClient.getProperties();
} catch (error) {
    if (parseError(error).errorType !== "404") { throw error; }
}

await blobClient.getBlockBlobClient().uploadData(content, {
    blobHTTPHeaders: {
        blobContentType: existingProperties?.contentType || mime.getType(uri.path) || undefined
    }
});

This matches the existing pattern in blobUtils.ts (getExistingProperties / createOrUpdateBlockBlob) already used by the AzureStorageFS code path.