ci: run the test suite as parallel sections by area

[ZhuchkaTriplesix]

Summary

The CI test job ran all 297 test files one after another on a single runner, so one slow or hung test held everything up. Tests now run as parallel jobs by area.

• scripts/ci/test_sections.sh defines the sections as sets of test directories:
  • core-data: database, storage, security, actions, app, json, csv, util
  • core-extensions: extensions, market, sdui, updater, features/extensions, features/updater
  • core-theme: theme, shared widgets
  • core-ui: motion, layout, ui, editor, widgets, platform, demo
  • features-workspace: workspace, sql_workspaces, sqlite, mysql, postgresql
  • features-nosql: mongodb, redis, nosql
  • features-app: main_screen, connections, settings, command_palette, onboarding, macos, help
  • rest: everything no section claims (so a test in a new directory still runs)
• ci.yml: a test-sections matrix runs flutter test --exclude-tags=golden <section paths> per section (fail-fast: false, so every section reports).
• The job named Run Tests is kept as the single aggregate check: it passes only when all sections and the coverage check pass, so branch rules and the PR checks list keep working. build-linux-release still depends on it.
• Test sections cover all tests runs scripts/ci/test_sections.sh check, which fails if a test file belongs to no section or to two.
• CONTRIBUTING.md explains how to run one section locally.

Notes

• Each section repeats Flutter setup and pub get (about a minute), so total runner time grows while wall-clock time drops to roughly the slowest section.
• A hung test still holds its own section until its timeout.