feat(ingest/superset): emit dataPlatformInstance for dashboards and charts

[Danishlynx]

Fixes #11153.

Superset has accepted platform_instance since #9647 but never emitted the dataPlatformInstance aspect, so the Navigate panel files every dashboard and chart under "Default" no matter how many deployments you ingest. The code carried a TODO for exactly this.

Changes

• superset.py: a get_data_platform_instance helper, modelled on Tableau's except that it returns nothing when platform_instance is unset, so existing deployments and the five existing goldens are untouched; the aspect appended at the dashboard and chart snapshot sites; the PLATFORM_INSTANCE capability declared. Preset builds on this source and inherits both with no code of its own; a unit test pins that its aspect resolves to urn:li:dataPlatform:preset.
• Tests: unit present/absent cases for dashboards and charts, an empty-string case, the Preset inheritance and capability cases, and one integration test with a new golden that sets platform_instance.
• Docs: a Platform instance section on both connector pages.
• The connector registry JSON regenerated (it gains the capability rows for superset and preset).

Datasets are deliberately not in this PR

Superset builds dataset URNs through get_datasource_urn_from_id, which uses plain make_dataset_urn, so those URNs carry no instance (the instance-aware gen_dataset_urn helper exists in the file but is unused). Emitting the aspect on datasets would name an instance the URN does not have, and two deployments exposing the same database, schema and table would overwrite each other's aspect on the one entity. Making the dataset URN instance-aware is a breaking URN change that also has to move the chart-to-dataset lineage edges, so it belongs with the containers follow-up rather than here. A unit test pins that datasets get no aspect, the TODO says why, and the docs say the same.

Notes for review

• Snapshot style kept: the aspect joins the existing DashboardSnapshot and ChartSnapshot aspect lists rather than going out as a separate MCP, matching how this source emits today. Containers in the follow-up will go out as MCP workunits beside the snapshots, the way Mode does.
• Goldens: the five pre-existing superset and preset goldens are byte-identical, which is the evidence that nothing changes when platform_instance is unset. The new golden carries exactly seven aspects (two dashboards, five charts).
• Follow-ups, in order: database and schema containers, the dataset instance question above, and test_connection.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues
• Tests for the changes have been added/updated
• Docs related to the changes have been added/updated

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Summary by cubic

Fixes #11153. Superset has accepted platform_instance since #9647 but never emitted the dataPlatformInstance aspect, so the Navigate panel filed every dashboard and chart under "Default". Now Superset and Preset emit the aspect on dashboards and charts when platform_instance is set; when it isn't, output is unchanged.

Datasets

• Datasets are excluded because their URNs don't carry an instance.
• Making dataset URNs instance-aware is a breaking change and will follow with the containers work.

Tests and docs

• Adds unit coverage for present, absent, empty-string, and Preset-inheritance cases plus one integration golden.
• Adds a Platform instance section to both connector docs and regenerates the connector registry with the PLATFORM_INSTANCE capability.

^{Written for commit 637466a. Summary will update on new commits.}