[Studio] Scope data source test loading by row

[Aias00]

Summary

• track data source connection-test loading by row key instead of a global boolean
• keep modal connection-test loading isolated from table row actions
• add regression coverage that only the clicked row shows loading

Verification

• npm --prefix web test -- --run src/pages/settings/tests/DataSourceTab.test.tsx
• (cd web && npm exec eslint -- src/pages/settings/index.tsx src/pages/settings/tests/DataSourceTab.test.tsx)
• npm --prefix web run build

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[Aias00]

Superseded by #643. Consolidated with related data source management changes to reduce review overhead while preserving the same scoped fixes and tests.