ci: add nightly integration tests for transformers, diffusers, peft, axolotl

Run each downstream library's own bnb-specific test suite against the
latest main bnb wheel (from the continuous-release_main pre-release).
Catches downstream breakage before it reaches users.

Jobs (all run in parallel):
- test-transformers (A10G): tests/quantization/bnb/ from transformers v5.x
- test-transformers-multigpu (4×L4): same, multi-GPU tests only [disabled
  until runner is provisioned]
- test-diffusers (L40S, Docker): pytest -m bitsandbytes across quantization
  + model tests, using diffusers/diffusers-pytorch-cuda container
- test-axolotl (A10G): kernel-level bnb tests (dequantize, LoRA, QLoRA)
- test-peft (L4): PeftBnbGPUExampleTests single-GPU per Benjamin's recommendation
- test-peft-multigpu (4×L4): same, multi-GPU [disabled until runner]
- report (ubuntu): consolidates JUnit XMLs, posts to #bnb-daily-ci-collab
  via Slack bot token with threaded per-suite failure details

Triggers: workflow_dispatch + pull_request (scoped to workflow/script changes).
Cron schedule commented out — enable in a follow-up PR once stable.

Also bumps the transformers test dep upper bound from <5 to <6, since
transformers v5.x is released and the bnb test suite there has fixes
(e.g. removed gated mosaicml/mpt-7b dependency).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Titus-von-Koeller