chore: update react to 19 and fix strict mode tests

[Wendy Fouchter (Wendystraite)]

Description of the change

Update React from 18 to 19.

Strict mode tests broke due to how strict mode changed in React 19 : useMemo and useCallback reuse the memoized results from the first render during the second render and since useMolecule calls useLazily inside a React useMemo, it prevents some unwanted executions.

See https://react.dev/blog/2024/04/25/react-19-upgrade-guide#strict-mode-improvements

React 19 includes several fixes and improvements to Strict Mode.

When double rendering in Strict Mode in development, useMemo and useCallback will reuse the memoized results from the first render during the second render. Components that are already Strict Mode compatible should not notice a difference in behavior.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation or Development tools (readme, specs, tests, code formatting)

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Latest commit: 0bee047

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[Copilot]

Pull Request Overview

Updates React from version 18 to 19 and adjusts test expectations to account for React 19's improved strict mode behavior, where useMemo and useCallback now reuse memoized results from the first render during the second render.

• Updated React and related dependencies from version 18 to 19
• Modified test expectations to reflect React 19's strict mode improvements where useMemo prevents duplicate executions
• Added sanity tests to document and verify the new React 19 strict mode behavior

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
package.json	Updated React dependencies from v18 to v19 and removed obsolete packages
src/react/useMolecule.test.tsx	Adjusted execution count expectations in strict mode tests from 2 to 1 due to useMemo improvements
src/react/ScopeProvider.test.tsx	Simplified strict mode test assertions to match new behavior where execution counts are consistent
src/react/sanity.test.tsx	Added new tests documenting React 19's useMemo and useCallback behavior in strict mode

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[Logan Volkers (loganvolkers)]

Wendy Fouchter (@Wendystraite) please update your PR on top of main to resolve merge conflicts.

[Wendy Fouchter (Wendystraite)]

Logan Volkers (@loganvolkers) all my PRs have been rebased.