Remove rounded border on top right and top left

[finnar-bin]

Removed rounded borders on the top right and top left of the global search dropdown
Resolves #3470

Preview

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Acceptance Criteria QA — ✅ PASS — 2/2 code-checkable criteria confirmed, 1 needs a running app

Validates #3470: Search: Dropdown - Rounded Borders appearing incorrectly

1. ✅ Top-left and top-right corners of the search dropdown paper are set to 0 radius
2. ✅ Bottom corners remain rounded (4px), matching the pre-existing intended design

Not checkable from the code — needs a running app

• Fix visually resolves the "rounded borders appearing incorrectly" bug in the browser — the change nests the radius override under a .MuiAutocomplete-paper.MuiPaper-rounded selector to win CSS specificity against MUI's default rounded-corner class, which is a plausible root cause, but this can't be confirmed without rendering the component

Suggested Cypress coverage

cypress/e2e/search/search-bar.spec.js has no assertions on dropdown appearance today (its cases cover saved keywords and accelerators). A visual/structural regression test could open the global search dropdown after typing a query, target the results panel via a data-cy selector, and assert computed border-top-left-radius/border-top-right-radius are 0px while the bottom corners are 4px, covering both the "has options" state (border applied on 3 sides) and the empty/no-results state (no border) so the corner styling can't silently regress again.

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Code Review — ✅ No blockers

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Adversarial Browser QA

No reproducible edge-case failures found on the surfaces this PR changes.

Also checked and working correctly

• GlobalSearch dropdown (src/shell/components/GlobalSearch/index.tsx) with an active results list (query "qa", "Baseline") — computed border-top-left-radius/border-top-right-radius are 0px and bottom corners stay 4px, matching the fix's intent.
• Same dropdown in its no-results state (query "zzzznonexistentqueryxyz123") and whitespace-only query (" ") — border-radius stays correctly squared on top, rounded on bottom in both cases.
• Empty-query "Search in..." chip-picker state (Content/Schema/Media/Code/Block shortcuts) — same corner treatment applies correctly.
• Clicking the native "Clear" (X) button to reset the field from a populated query back to the chip-picker state — reverts cleanly, no leftover stale option rows.
• Clicking a search result ("NegQA Baseline With Delay") — navigates to the correct content item edit route; the new nested &.MuiAutocomplete-paper.MuiPaper-rounded selector doesn't interfere with option click-through.
• An item name containing <script>alert(1)</script> in the result list renders as literal escaped text, not executed — pre-existing, unaffected by this change.

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Code Review — ✅ No blockers

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Coverage Report

Overall Coverage

Lines	Statements	Branches	Functions
61.76%	61.38%	55.96%	56.35%

Changed Files Coverage

File	Lines	Statements	Branches	Functions
src/shell/components/GlobalSearch/index.tsx	80.52%	80.92%	69.40%	86.00%