[diffusion]: Fix diffusers executor crash when component residency manager is absent

[qimcis]

Summary

Fix a --backend diffusers runtime crash introduced by the component residency manager refactor in #23771.

DiffusersPipeline uses the common SyncExecutor, but it was not binding a component residency manager before executing stages the way composed/native pipelines do. After the refactor, the shared executor calls residency hooks such as component_residency_manager.begin_request(...). For diffusers pipelines, that manager was None, so generation failed before the diffusers pipeline ran.

This change initializes the diffusers pipeline residency state and binds the global component residency manager to the executor before forwarding the request

Motivation

Forced diffusers backend models can load successfully but fail on the first generation request:

AttributeError: 'NoneType' object has no attribute 'begin_request'

Testing

1x rtx5090 32GB
black-forest-labs/FLUX.2-klein-4B

Baseline command:

sglang generate \
  --model-path black-forest-labs/FLUX.2-klein-4B \
  --backend diffusers \
  --trust-remote-code \
  --prompt "A compact engineering test image of a red cube on a gray table, studio lighting" \
  --height 512 \
  --width 512 \
  --num-inference-steps 1 \
  --guidance-scale 1.0 \

the pipeline loads:

Loaded diffusers pipeline: Flux2KleinPipeline

but the generation request fails before the diffusers stage runs:

Error executing request ...: 'NoneType' object has no attribute 'begin_request'
AttributeError: 'NoneType' object has no attribute 'begin_request'

After fix command:

sglang generate \
  --model-path black-forest-labs/FLUX.2-klein-4B \
  --backend diffusers \
  --trust-remote-code \
  --prompt "A compact engineering test image of a red cube on a gray table, studio lighting" \
  --height 512 \
  --width 512 \
  --num-inference-steps 1 \
  --guidance-scale 1.0 \

the pipeline loads:

Loaded diffusers pipeline: Flux2KleinPipeline

And the generation request succeeds:

Loaded diffusers pipeline: Flux2KleinPipeline
[DiffusersExecutionStage] finished
Output saved to outputs/flux2_klein_diffusers.png
Pixel data generated successfully

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[qimcis]

maybe we need to add some --backend diffusers ci? i think if i rebase and improve #19213 it would be enough @mickqian

[qimcis]

/tag-and-rerun-ci

[mickqian]

duplicated with #24748