[ci] Repair fastcheck contract drift

[Mister-Raggs]

Problem

Fastcheck currently has several deterministic failures on main that were exposed when #1727 changed pyproject.toml and therefore triggered every fastcheck lane:

• [bugfix]: hard-fail when ATTN_QAT_INFER is selected but the kernel is unusable #1690 made an explicitly selected but unusable ATTN_QAT_INFER backend fail loudly, while the duplicate API capability tests still expected silent fallback.
• New multi-image-encoder and MMAudio pipeline fields were not classified in the inference schema parity inventory.
• The new mlx and platforms test directories were neither collected nor explicitly allowlisted.
• FA4 policy tests matched an exact one-line dictionary layout and broke when formatting wrapped the entries, although the runtime defaults remained correct.

Changes

• Align the API capability tests with the intentional ATTN_QAT_INFER hard-fail contract.
• Classify all current PipelineConfig base and extension fields.
• Add the CPU-safe platforms tests to the unit lane and explicitly document why the Apple/Metal-specific mlx suite is not run on Linux CUDA Modal workers.
• Inspect FA4 image environment defaults through the Python AST instead of formatter-sensitive source substrings.

Validation

• Focused Modal L40S run: 32 passed in 1.49s
  • https://modal.com/apps/hao-ai-lab/main/ap-IIXGFLo9v0R7H5gm55NPhR
• Complete unit pytest selection on Modal L40S: 900 passed in 62.31s
  • https://modal.com/apps/hao-ai-lab/main/ap-YIhP7DVneoqEeyHAomjTvL
  • Used the existing CI image without rebuilding kernels because this patch changes only tests, inventory metadata, and test collection.
• pre-commit run --all-files: all hooks passed, including mypy.

GPU memory impact

None. This changes test expectations, schema inventory metadata, and CI test collection only.

Context

Discovered while investigating unrelated fastcheck failures on #1727. The failing implementation and test files were byte-for-byte identical to upstream main before this repair.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[mergify]

Merge Protections

🔴 1 of 1 protections blocking · waiting on 👀 reviews and 🤖 CI

	Protection	Waiting on
🔴	PR merge requirements	👀 reviews and 🤖 CI

🔴 PR merge requirements

Waiting for

• #approved-reviews-by>=1
• check-success=full-suite-passed

This rule is failing.

• #approved-reviews-by>=1
• check-success=full-suite-passed
• check-success=fastcheck-passed
• check-success~=pre-commit
• title~=(?i)^\[(feat|feature|bugfix|fix|refactor|perf|ci|doc|docs|misc|chore|kernel|new.?model|skill|skills|infra)\]

[mergify]

This PR has merge conflicts with the base branch. Please rebase:

git fetch origin main
git rebase origin/main
# Resolve any conflicts, then:
git push --force-with-lease

[SolitaryThinker]

Closing as superseded by merged #1747. I verified that current main contains the schema classifications and explicit ATTN_QAT_INFER fail-closed behavior, replaces the Modal unit collection change with the active Slurm/macOS lane topology, and retains syntax-insensitive FA4 policy checks. Current-main validation passed: 27 capability/CI contract tests and 13 schema-parity tests. Rebasing this patch would duplicate or regress the current CI structure.