fix(BA-2370): Handle Storage Proxy connection error

[fregataa]

resolves #5867 (BA-2370)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull Request Overview

This PR adds error handling for storage proxy connection failures by introducing a new exception class and catching connection errors during volume retrieval operations.

• Introduces StorageProxyConnectionError exception class for handling storage proxy connectivity issues
• Updates the storage proxy manager client to catch and re-raise connection errors as the new exception type
• Modifies the session manager to gracefully handle connection failures by logging warnings and returning empty results

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/ai/backend/manager/errors/storage.py	Adds new StorageProxyConnectionError exception class
src/ai/backend/manager/clients/storage_proxy/session_manager.py	Imports new exception and handles connection errors in volume fetching
src/ai/backend/manager/clients/storage_proxy/manager_facing_client.py	Catches aiohttp.ClientConnectionError and re-raises as StorageProxyConnectionError

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[Copilot]

The HTTP status code should be 503 Service Unavailable (web.HTTPServiceUnavailable) instead of 400 Client Error (web.HTTPClientError) since connection failures indicate the service is temporarily unavailable, not a client request error.

Suggested change

	class StorageProxyConnectionError(BackendAIError, web.HTTPClientError):
	class StorageProxyConnectionError(BackendAIError, web.HTTPServiceUnavailable):

[HyeockJinKim]

It seems like it is not a ClientError because it is a Server Connection Error.

[HyeockJinKim]

It seems like it is not a ClientError because it is a Server Connection Error.

[HyeockJinKim]

It doesn't seem like this is the way to consume the error, is there a reason you worked this way?