CI: simplify onboard scene workflows

[luohuan19]

Summary

• remove the legacy path-based a2a3 SDMA mode from the reusable NPU scene-test workflow
• make both CI and CI Self CPU use the marker-based SDMA split
• remove the redundant graphviz warning step
• split onboard NPU DFX smokes into dedicated main-CI reusable workflows, so CI Self CPU no longer carries NPU DFX skip-only steps
• keep the dedicated SDMA step while eliminating the legacy-mode branch and NPU DFX toggle from the base onboard scene workflow

Relation to #1639

#1639's main dedupe is already present on main: ci.yml and ci-self-cpu.yml share the canonical _detect-changes.yml, common job bodies are reusable workflow_call workflows, and the pip cache / venv scaffolding lives in composite actions.

This PR finishes the remaining cleanup from that direction by removing the legacy a2a3 SDMA path mode, the redundant graphviz warning step, and the onboard NPU DFX toggle that made CI Self CPU carry skip-only DFX steps.

Checkout intentionally remains inline as actions/checkout@v5 inside the reusable workflows. A local composite action cannot safely wrap the first checkout step because local actions are loaded from the workspace after checkout has already happened. For the same reason this PR does not add a policy check that bans inline checkout usage.

No new structural guard check is added here. The high-risk duplication has been removed structurally; adding a string-scan policy check would expand this cleanup PR and create another maintenance point for normal workflow renames or job reshaping.

Validation

• parsed all GitHub workflow YAML files with PyYAML
• ran git diff --check
• ran actionlint v1.7.12 on touched workflow files
• full actionlint only reports pre-existing custom self-hosted label warnings in ci-self-cpu-button.yml and ci-self-cpu-report.yml

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📝 Walkthrough

Walkthrough

The A2A3 CI workflows remove the a2a3_sdma_mode input. Scene tests use the A2A3 platform condition and exclude SDMA-marked tests. The SDMA step always runs SDMA-marked tests. Legacy SDMA-path steps are removed.

Changes

A2A3 SDMA CI workflow

Layer / File(s)	Summary
A2A3 workflow conditions .github/workflows/_st-npu.yml, .github/workflows/ci-self-cpu.yml, .github/workflows/ci.yml	The workflow callers no longer pass a2a3_sdma_mode. A2A3 scene tests and SDMA-marked tests now run through fixed conditions, without legacy SDMA-path branching.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• hw-native-sys/simpler#1476: Modifies the st-onboard-a2a3 workflow configuration in ci.yml.
• hw-native-sys/simpler#1609: Introduces marker-based SDMA test isolation.
• hw-native-sys/simpler#1637: Modifies A2A3 SDMA marker filtering and workflow behavior.

Poem

A rabbit checks the A2A3 run,
Scene tests hop beneath the sun.
SDMA tests take their steady turn,
No legacy paths remain to learn.
Clean workflows leap ahead! 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the workflow simplification described in the changeset.
Description check	✅ Passed	The description directly explains the A2A3 SDMA workflow cleanup and related CI changes.

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