[MAX] Add v2 Qwen3 text encoder for diffusion pipelines

[pei0033]

Summary

Adds a v2 qwen3/text_encoder implementation for diffusion pipelines.

This migrates the Qwen3 text encoder off the module v3 max.experimental path and keeps the file hierarchy and code order aligned with the existing module v3 implementation to simplify review and follow-on changes.

Testing

• ./bazelw run //max/examples/diffusion:simple_offline_generation --config=disable-mypy -- --model black-forest-labs/FLUX.2-klein-4B --prompt 'a small test image of a cat' --height 256 --width 256 --num-inference-steps 1 --num-warmups 0 --guidance-scale 1.0 --seed 42 --output v2.png

Checklist

• PR is small and focused — consider splitting larger changes into a
  sequence of smaller PRs
• I ran ./bazelw run format to format my changes
• I added or updated tests to cover my changes
• If AI tools assisted with this contribution, I have included an
  Assisted-by: trailer in my commit message or this PR description
  (see AI Tool Use Policy)

Assisted-by: Codex

[ansley]

!sync

[modularbot]

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[modularbot]

Landed in 18fb0e9! Thank you for your contribution 🎉