test(subdirectory): scaffold red/green tests for application root URL helpers

[sadpandajoe]

SUMMARY

Skeleton commit for a TDD-driven refactor that makes subdirectory deployments (SUPERSET_APP_ROOT) decision-free for frontend developers. After the full PR lands, every URL in the codebase is router-relative; the channel-3 helpers (openInNewTab, redirect, getShareableUrl, <AppLink>) wrap internally, and a backend response normaliser strips the application root on the way in so frontend code only ever speaks one shape.

This PR is intentionally just the scaffolding so the framing can be reviewed before ~30-60 call sites get migrated.

What's in this PR

Frameworks (implemented)

• spec/helpers/withApplicationRoot.ts — fixture that sets application_root in the bootstrap data, resets the module cache, and tears down on completion. Replaces an inline ritual that pathUtils.test.ts repeats per test.
• spec/helpers/sourceTreeScanner.ts — line-by-line regex scanner over the source tree with allow-list support. Reports workspace-relative file:line for each violation.

Stubs (throw "not implemented")

• src/utils/navigationUtils.ts — openInNewTab, redirect, redirectReplace, getShareableUrl, AppLink. Each documents the channel rule it enforces. Existing navigateTo / navigateWithState stay as legacy multi-mode helpers and are scheduled for replacement.
• packages/superset-ui-core/src/connection/normalizeBackendUrls.ts — conservative URL field normaliser. Ships a curated NORMALIZED_URL_FIELDS set (initially empty pending per-endpoint audit) and a documented NORMALIZER_EXCLUSIONS list explaining why fields like bug_report_url, thumbnail_url, and user_login_url are deliberately not normalised.

Layered tests (one example each; expanded in follow-up commits on this PR)

Layer	What it covers	Example file	State on day 1
1	Per-helper unit behaviour under empty / single / nested app roots	navigationUtils.test.ts (openInNewTab)	🔴 red — helper throws
2	Static invariant: no ensureAppRoot / makeUrl import outside navigationUtils.ts	navigationUtils.invariants.test.ts	🟢 green — allow-list seeded with the 19 current call sites
3	Backend URL normaliser: positive strip + negative passthrough cases	normalizeBackendUrls.test.ts	🔴 red — normaliser throws
4	SupersetClient × applicationRoot contract: root applied exactly once	SupersetClientAppRootContract.test.ts	🟢 green — formalises existing behaviour
5	Per-site regression for SliceHeaderControls Cmd-click "Edit chart"	SliceHeaderControls.subdirectory.test.tsx	🔴 red — index.tsx:266 calls window.open(props.exploreUrl) without prefixing

Why this approach

The codebase has three URL channels — React Router, SupersetClient, and browser-direct sinks (window.open, window.location.href, <a href>). Today each has its own convention for whether the caller pre-applies the application root, and bugs come from mixing them up:

• Double-prefix: passing an ensureAppRoot(...) value into a router or SupersetClient channel that already applies the prefix internally.
• Missing-prefix: passing a raw /path to a browser-direct sink that does not.

The fix makes router-relative the canonical shape — the one developers always write — and pushes the wrapping decision into helpers so individual call sites never need to think about it. Combined with the response normaliser, backend-supplied URLs (explore_url, permalink, etc.) speak the same shape, and ensureAppRoot / makeUrl become module-private inside navigationUtils.ts.

Strategy

Each subsequent commit on this PR fans out one layer to its full coverage and migrates the corresponding call sites, shrinking the Layer 2 allow-list in lockstep. Final commit lands an ESLint rule that complements the static-invariant tests.

When the in-flight subdirectory branches (origin/subdirectory-bugs, origin/subdirectory-export) merge to master, this branch rebases onto them and adapts.

TESTING INSTRUCTIONS

Run the test suite from superset-frontend/:

npm run test -- spec/helpers/withApplicationRoot \
                src/utils/navigationUtils.invariants.test.ts \
                packages/superset-ui-core/test/connection/SupersetClientAppRootContract.test.ts

Layers 2 and 4 should pass on day one. Layers 1, 3, and 5 are intentionally red — they describe the behaviour the green commit will deliver.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration
• Introduces new feature or API
• Removes existing feature or API

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