[#3] feat: add --filter option to export-assets for selective dashboard export (#377)

* feat: add --filter option to export-assets for selective dashboard export

Adds a repeatable --filter / -t option to the export-assets command
that restricts which dashboards are exported. Filters are expressed as
key=value pairs and ANDed together.

Supported dashboard filter keys: id, slug, dashboard_title,
certified_by, is_managed_externally (alias: managed_externally).
dashboard_title uses substring (contains) matching; all others use
exact matching. When the Superset API does not support a filter field,
results are fetched unfiltered and matched locally.

Builds on the shared filter infrastructure introduced in
feature/filter-infrastructure.

Co-Authored-By: Richard Fogaca <richard@preset.io>

* fix: update export command help snapshot to include new filter options

* Update src/preset_cli/cli/superset/export.py

Co-authored-by: Enzo Martellucci <52219496+EnxDev@users.noreply.github.com>

* refactor: extract superset export helpers module

* test(export): cover export helper defensive branches

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Co-authored-by: Richard Fogaca <richard@preset.io>
Co-authored-by: Enzo Martellucci <52219496+EnxDev@users.noreply.github.com>

Author: 3 people