Refactor FilesExt: centralize hostname routing and simplify session management

[parthban-db]

🥞 Stacked PR

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• stack/refactor-files-client-2 [Files changed]
  • stack/refactor-files-client-3 [Files changed]

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Why

Today the FilesExt class builds API URLs using hardcoded relative paths (e.g. "/api/2.0/fs/create-upload-part-urls") and creates a new unauthenticated requests.Session on every upload/download operation. This works, but has two limitations:

• No single point of control for request routing. When we add storage-proxy support, uploads need to be routed to a different hostname. With paths scattered across a dozen methods, everyone needs to change. A central _get_hostname() makes the future change a one-liner.
• Unnecessary session churn. A cloud_provider_session was created by a high-level method and threaded through 5+ layers of calls as a parameter. Some methods received it only to pass it further down. This made signatures noisy, and the data flow hard to follow.

What changed

Interface changes

None. All changes are to private methods.

Behavioral changes

None. This is a pure refactor — the cloud provider session is now cached and reused instead of re-created per operation, but requests.Session is safe to share across calls.

Internal changes

• _get_hostname() — new method returning the workspace hostname. All self._api.do() calls now use url=f"{hostname}/api/..." instead of relative paths. This is the future extension point for storage-proxy routing.
• _cloud_provider_session() — renamed from _create_cloud_provider_session(). Caches the session on first call. Callers invoke self._cloud_provider_session() directly instead of storing a local variable.

How is this tested?

• All existing unit tests.
• One new test case was added ("Create resumable URL: node exists but URL is empty") to cover the build_resumable_upload_url validation path, where the response node exists but contains no URL.

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• PR number: 1294
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[yuanjieding-db]

The self._cached_cloud_provider_session is cached forever once it's created. I am a bit concerned about this as in rare cases the Session might enter an unrecoverable state and all of the following requests will fail and the entire WorkspaceClient has to be re-created to recover.

Example of such case (though this particular case has already been fixed): link

[yuanjieding-db]

There are a few possible ways to avoid this scenario:

• Automatic discard of cache: Maybe we can clear this cache after certain time (say 1h)
• Active discard of cache by caller: We can also add this logic to the retry function to make sure the cache is cleared if all retry has failed.

[yuanjieding-db]

Can we make this function atomic? Multiple threads may be calling the function at the same time and it's possible to create multiple new Sessions while only one Session is cached and returned.