fix(BA-4392): Share single Docker client across container stats collection

[seedspirit]

resolves #8809 (BA-4392)

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

Refactors Docker-based container stats collection to reuse a single aiodocker.Docker() client per collection pass, reducing file descriptor pressure in large deployments.

Changes:

• Update CPUPlugin.gather_container_measures() to reuse one Docker client for all per-container API stats calls.
• Update MemoryPlugin.gather_container_measures() to reuse one Docker client for both API and sysfs-derived stats paths.
• Add unit tests asserting only a single Docker client is created per collection pass, plus a changelog entry.

Reviewed changes

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

File	Description
src/ai/backend/agent/docker/intrinsic.py	Refactors CPU/memory container stats to share a single Docker client instance during per-pass collection.
tests/unit/agent/test_docker_intrinsic.py	Adds tests verifying Docker client instantiation count in CPU/Memory plugins across stat modes.
changes/9469.enhance.md	Documents the FD-exhaustion mitigation by sharing a Docker client during stats collection.

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[fregataa]

It would be nice to write summary description.
Do we have Docker daemon infra for unit test? @HyeockJinKim

[github-actions]

Backport to 25.15 is failed. Please backport manually.