[Config] Migrate weight-offloading env vars to OffloadConfig

[halfrost]

Purpose

Part of RFC #25700 (limit the use of env vars in vLLM). Environment
variables lack type safety, CLI documentation, and hierarchical grouping, and
are the path of least resistance for new flags. This migrates the two
weight-offloading toggles to first-class config under the existing
OffloadConfig, following the same pattern used for the attention-config
migration (#26315).

Deprecated env var	New config	New CLI flag
VLLM_WEIGHT_OFFLOADING_DISABLE_PIN_MEMORY	OffloadConfig.disable_pin_memory	--offload-disable-pin-memory
VLLM_WEIGHT_OFFLOADING_DISABLE_UVA	OffloadConfig.uva.disable_uva	--offload-disable-uva

Design

• Both new fields default to None, meaning "fall back to the deprecated env
  var".
• resolve_offload_flag() (vllm/model_executor/offloader/base.py) centralizes
  the precedence rule: an explicitly set config value always wins; otherwise the
  deprecated env var is used, emitting a one-time deprecation warning.
• should_pin_memory() and UVAOffloader read through it;
  model_loader/utils.py now routes its pin-memory check through
  should_pin_memory() instead of reading the env var directly.

Backward compatibility

The env vars are kept and marked deprecated in vllm/envs.py. Existing
deployments that set them keep working unchanged (with a deprecation warning);
the new CLI flags / config take precedence when provided.

Not a duplicate

Checked open PRs (gh pr list --search "WEIGHT_OFFLOADING", "25700 in:body",
"disable_pin_memory offload") — no existing PR migrates these env vars to
config. The offload-related open PRs (#43453, #37190) address different
functionality.

Test plan

• Added tests/model_executor/offloader/test_weight_offload_config.py
  (cpu_test): covers config-over-env precedence, env fallback when the config
  field is unset, the one-time deprecation warning, and the default-off path.

pytest tests/model_executor/offloader/test_weight_offload_config.py -v
pre-commit run --all-files

Draft status: kept as a draft until the author runs the full local test
suite + pre-commit on GPU hardware and pastes the results here. The change
set was syntax-checked and the new unit test is self-contained
(cpu_test), but has not yet been executed in a full vLLM environment.

Note

AI assistance was used to prepare this change; every line was reviewed by the
submitter per AGENTS.md.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @halfrost.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork