[BUG] [Cowork] Windows: Excel "open in app" fails silently when generated path exceeds MAX_PATH (preview still works)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] We are on the current version of Cowork in the cloud (cloud-hosted, so version is whatever is currently deployed)

What's Wrong?

On Windows, Excel files in a Cowork in the cloud session cannot be opened directly in the desktop Excel app with a single click. The open-in-app action fails silently — no error dialog, no toast, nothing. The in-platform preview renders the same file correctly, which makes the file look fully accessible when it isn't.

Root cause (as best we can determine): the local file path generated by Cowork's directory structure exceeds 260 characters, the MAX_PATH ceiling enforced by the Windows file path APIs. The OS rejects the path before Excel is ever invoked, so the failure surfaces nowhere.

Two aggravating details:

1. Enabling Windows long-path support does not fix it. The LongPathsEnabled registry policy only helps applications that declare longPathAware in their manifest — Microsoft Office does not. So there is no user-side or IT-side workaround via OS configuration.
2. Excel's practical limit is lower than 260. Excel historically fails on full workbook paths longer than ~218 characters. A fix that only targets the 260-char ceiling would still leave a failure band.

What Should Happen?

Either of:

• Clicking an Excel file in Cowork opens it in desktop Excel (paths generated by Cowork stay under Excel's ~218-char practical limit — e.g. by flattening the generated directory structure or hashing long path segments), or
• At minimum, the failure is surfaced to the user with an actionable error ("path too long for Excel — download the file or shorten the folder name") instead of failing silently while the preview implies the file is fine.

Steps to Reproduce

1. On Windows, use Cowork in the cloud in a workspace whose generated directory structure plus file name yields a local path longer than ~218 characters (deep folder nesting under the Cowork-generated root is enough).
2. Observe the file previews correctly in the platform.
3. Click to open the file in the desktop Excel app.
4. Nothing happens — no Excel launch, no error.
5. Shorten the path (move the file up the tree or rename folders) below the limit and the same action works.

Additional Information

• Environment: Windows, Cowork in the cloud, Microsoft 365 desktop Excel.
• Scale in our org: small deployment (17 seats, 4 active this month), but we believe the bug is underreported rather than rare — the working preview teaches users the file is accessible, so they blame Excel or silently fall back to download-and-open rather than filing a ticket.
• LongPathsEnabled state is irrelevant to the failure (see above) — mentioning to preempt it as the suggested remediation.
• Happy to provide a concrete failing path with character counts from an affected machine if useful.