fix(gooddata-sdk): support tabbed dashboard layouts in translation methods

[vondravl]

https://gooddata.atlassian.net/browse/PSDK-230

Summary

• Dashboards can now use a tabbed layout where sections live under content.tabs[*].layout.sections instead of the legacy content.layout.sections
• get_texts_to_translate and set_translated_texts crashed with KeyError: 'layout' on dashboards that only use tabs (no top-level layout key)
• Adds _iter_dashboard_sections() helper that yields sections from both structures (legacy, tabbed, or both)
• Extracts date filter titles and tab titles from tab-level configs
• Includes 16 unit tests covering all layout variants (legacy-only, tabs-only, both, empty, malformed tabs)

Background

GoodData introduced a tabs array in dashboard content. Some dashboards now have:

{
  "tabs": [
    {
      "title": "Clearing",
      "layout": { "sections": [...] },
      "dateFilterConfig": { "filterName": "..." }
    },
    {
      "title": "Authorization",
      "layout": { "sections": [...] }
    }
  ]
}

Instead of (or in addition to) the legacy:

{
  "layout": { "sections": [...] }
}

The SDK assumed content["layout"] always exists, which fails on tab-only dashboards.

Test plan

• 16 unit tests added covering:
  • _iter_dashboard_sections: legacy-only, tabs-only, both, empty, tabs-without-layout
  • get_texts_to_translate: tabs-only, legacy, rich text in tabs, viz switcher in tabs, already-translated exclusion
  • set_translated_texts: tab title translation, widget translation in tabs, legacy still works
  • _extract_dashboard_date_filter_titles: top-level, tab-level, both
• Tested end-to-end against GoodData production workspace with 34 dashboards (3 tab-only)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.26%. Comparing base (0ddff14) to head (a2f761d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1525      +/-   ##
==========================================
+ Coverage   78.18%   78.26%   +0.07%     
==========================================
  Files         228      228              
  Lines       14930    14950      +20     
==========================================
+ Hits        11673    11700      +27     
+ Misses       3257     3250       -7     

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