fix(agent): save the kernel registry from a snapshot, not the live map

[daemyung-lablup]

save_kernel_registry iterates the live kernel_registry map and awaits once per kernel. A kernel created on the same node in between registers itself into that map, and the save raises RuntimeError: dictionary changed size during iteration inside the creating call -- which then fails the kernel it was creating. Measured with 27 two-node sessions started at once on three nodes: 1 of 27 failed on exactly this.

Change	Why
Iterate a snapshot of the registry	A kernel that arrives mid-save is written by the save its own create runs.
Commit a record only while the id still maps to the snapshot's object	A restart re-creates a kernel under the same id and keeps its directory; the old object's REPL ports must not land there. The record is staged beside recovery.json and renamed into place in one step, with no await between the check and the rename -- which also means a crash mid-write cannot leave a torn record.
Never create the config directory from a save	It is the kernel's own, removed with it; its absence means the kernel is gone, and re-making it left an empty directory behind. A failed or cancelled write removes its staged file.
Skip a PREPARING kernel quietly, keep the traceback for any other	Registered a step before its REPL ports exist, a PREPARING kernel is ordinary under concurrent creates. A RUNNING kernel without them is a broken invariant.

Independent of #14551, which fixes the periodic reconciler destroying a just-created kernel; no file overlap.

Tests: tests/unit/agent/kernel_registry/test_writer_container.py (mid-save registration, same-id replacement, destroyed-while-saving, PREPARING vs RUNNING), tests/unit/agent/test_scratch_config.py (stage/commit/discard, no directory creation, cleanup on failed serialization and on cancellation)

Checklist: (if applicable)

• Backport targets: a fix: PR goes to every version in .github/maintained-versions.yml automatically.
  A Backport: line here names the targets instead — some of them, other ones, or none.
• 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

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