fix(pages-router): only exclude top-level pages/api from page routing

[JaredStowell]

Motivation

• A recent change used the function-form glob exclude predicate which compared basenames and unintentionally excluded any directory named api at any depth, dropping valid nested page routes like pages/blog/api/index.tsx.
• The function-form exclude was necessary for Node < 22.14 compatibility, so the fix must preserve that compatibility while restoring correct route discovery.

Description

• Adjusted the scanPageRoutes glob usage to only exclude files/dirs that start with _ via the function-form predicate, removing the blanket name === "api" check.
• Added an explicit runtime check to skip only top-level pages/api/* entries (if (file === 'api' || file.startsWith( api${path.sep})) continue;) so nested */api/* directories remain discoverable.
• Added a fixture tests/fixtures/pages-basic/pages/blog/api/index.tsx and a test assertion in tests/routing.test.ts to cover discovery of nested api directories.
• Files modified: packages/vinext/src/routing/pages-router.ts, tests/routing.test.ts, and the new fixture file under tests/fixtures/pages-basic/pages/blog/api/index.tsx.

Testing

• Ran the targeted routing tests with npx vitest run tests/routing.test.ts, which passed: 1 test file, 61 tests, 61 passed.
• The change is focused and validated by the added fixture and test; broader test suites (full Vitest / Playwright) are left to CI.

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Codex Task