Add section_collapsed param to control default accordion state

[fenix7]

Summary

• Add section_collapsed boolean property to the Param schema, allowing DAG authors to control which sections in the trigger form start collapsed
• Sections with section_collapsed=True on their first param start collapsed; others start expanded
• When no section uses the flag, existing default behavior is preserved (only "Run Parameters" expanded)
• Add multiple to Accordion.Root so multiple sections can be open simultaneously
• Demonstrate the feature in example_params_ui_tutorial.py on the "Special advanced stuff with form fields" section

Test plan

• Run frontend unit tests (pnpm test) — 92/93 passing (1 pre-existing failure unrelated to this change)
• Open the trigger form for the example_params_ui_tutorial DAG and verify "Special advanced stuff with form fields" starts collapsed
• Verify all other sections start expanded by default
• Verify sections can be manually expanded/collapsed by clicking
• Verify that DAGs without section_collapsed retain the original behavior

[boring-cyborg]

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