Make changes to options when exporting in CSV and TSV

[chippison]

Description

DEV-14698

When exporting a report in CSV and TSV, we would want to:

• Remove/Hide the 'Expand subtables' option
• Pre-select 'Flatten report'

For other formats ('JSON', 'RSS', 'XML', 'HTML')

• We always show 'Expand subtables' option
• Initially this will be ticked
  • First time you select a report format above, the 'Expand subtables' option is ticked.
  • This will 'remember' what your choice was, and will reset the 'Expand subtables' option to this value when ever you select a report format that allows this option until you change it again
• This is an enforced UI change so that we can show this option without pre-selecting it when report format is changed from default ('TSV' where this option is hidden) to a report format which allows 'Expand subtables' option

NOTE:
CSV/TSV exports do not support expanded subtables, so we always export the flattened table for these formats. This changes the output from top‑level referrers to flattened subtable rows and removes goal‑specific columns that aren’t present in the flattened export. The new fixtures (tests/UI/expected-screenshots/ReportExporting_Referrers.getWebsites_exported.csv.txt and tests/UI/expected-screenshots/ReportExporting_Referrers.getWebsites_exported.tsv.txt) reflect the intended CSV/TSV behavior.

Acceptance Criteria

Should only happen when a datatable to be exported has subtables. We don't change behaviour when table has no subtable

• Test that when CSV or TSV format is chosen as the export format, 'Expand subtables' option should not be shown and 'Flatten report' is selected
• 'Expand subtables' should be shown and selected if it is any other option is chosen.

Checklist

• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated