Add displayMode fix for table widgets on entity pages

[gilmor-port]

Summary

• Adds client-side normalization in the validate-pages fix script to set displayMode: "widget" on table-entities-explorer widgets inside dashboard-widget containers
• Runs this fix proactively on every page before validation, so orgs with strict page validation (e.g. GitHub) can save table filters without 422 errors from sibling widgets missing displayMode
• Adds unit tests for the normalization logic and updates fix flow docs

Test plan

• npm test in validate-pages passes
• Run npm run fix against a test org with entity pages containing table widgets missing displayMode
• Re-run npm run validate and confirm affected pages are valid
• Verify saving a filter on a previously affected table widget succeeds in the UI

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