chore(scripts): validate react-native export conditions in yarn constraints

[udit (UditDewan)]

Platforms Impacted

• iOS
• macOS
• win32 (Office)
• windows
• android

Description of changes

Addresses #3936 by enforcing the "react-native" export-condition pattern in package.json files.

Metro resolves the react-native export condition to TypeScript source. Nothing validated those targets, and one was already broken: @fluentui-react-native/tester pointed react-native at ./src/index.ts, but the entry file is ./src/index.tsx. Its default condition already uses .tsx.

scripts/src/tasks/constrain.ts adds ensureReactNativeExports, run as part of yarn constraints (and therefore yarn lint-repo) for every non-private workspace:

• Each exports entry's "react-native" target must exist. If only the .ts/.tsx extension is wrong, yarn constraints --fix corrects it; otherwise it reports an error.
• Packages that declare react-native in peerDependencies must provide a "react-native" condition on every export. All current RN libraries already comply, so this only guards new packages.
• Node-only tooling (desktop-driver, storybook-desktop, scripts) and private apps are not affected.

apps/fluent-tester/package.json gets the fix from yarn constraints --fix, plus a patch changeset.

Verification

• yarn constraints on main with the new rule reports exactly one error:
  @fluentui-react-native/tester: Invalid field exports["."]["react-native"]; expected "./src/index.tsx", found "./src/index.ts". After --fix it is clean.
• Temporarily removing react-native from @fluentui-react-native/menu's exports and adding a bogus ./x entry produced both new errors (is missing a "react-native" condition, points to ./src/nope.ts, which does not exist). The edit was reverted afterward.
• yarn build, yarn lint in scripts, yarn format:check, yarn change:check.
• yarn knip fails identically on main (pre-existing unused-dependency findings); this change removes its apps/fluent-tester "Package entry file not found" hint.

Before	After
react-native export targets are unchecked; tester's points at a missing file	Constraint validates/fixes targets and requires the condition for RN libraries

Pull request checklist

This PR has considered (when applicable):

• Automated Tests
• Documentation and examples
• Keyboard Accessibility
• Voiceover
• Internationalization and Right-to-left Layouts