feat(BA-3948): Add client SDK support for events APIs

[rapsealk]

Resolves #8131 (BA-3948)

This pull request introduces a new client SDK Events class and associated CLI commands to enable real-time streaming of session/kernel lifecycle events and background task events via Server-Sent Events (SSE). It also integrates the Events API into the session client and makes some minor dependency import fixes.

Event Streaming Functionality

• Added a new Events class in src/ai/backend/client/func/events.py to provide async methods for subscribing to session and background task events using SSE.
• Registered the Events API in the AsyncSession class, making it accessible via session.Events. [1] [2] [3]

CLI Enhancements

• Introduced new CLI commands under the events group in src/ai/backend/client/cli/events.py:
  • listen-session: Stream session/kernel events with filtering and wait options.
  • listen-background-task: Stream background task events for a given task ID. [1] [2]

Documentation

• Added a .feature.md entry describing the new event streaming capabilities for both SDK and CLI.

Dependency Import Fixes

• Fixed import of ClosingError in valkey_client/client.py and TimeoutError in valkey_stream/client.py to use the correct module paths. [1] [2] [3]

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