fix: fail when exact template version does not exist

[fabriziocucci]

Description

Problem:
When running npx @react-native-community/cli init MyApp --version "0.83.3", if the template for version 0.83.3 doesn't exist but 0.83.2 exists, the CLI would silently fall back to 0.83.2 instead of failing.

This caused real problems:

1. rn-diff-purge breakage: The release job would succeed even when the requested template version didn't exist, because the CLI silently used a different template. This led to incorrect diffs being generated.

2. Confusing user experience: Users would see Welcome to React Native 0.83.3 in the console output, but when they checked their package.json, they'd find a different version. This mismatch is confusing and can lead to debugging headaches.

Solution:
Removed the minor version fallback logic in getTemplateVersion(). The CLI now only matches exact MAJOR.MINOR.PATCH versions. If the requested version doesn't have a corresponding template, an error is thrown with a clear message.

Changes

File	Change
packages/cli/src/tools/npm.ts	Removed minor version fallback logic and unused minorVersion helper
packages/cli/src/commands/init/version.ts	Added error handling when template version is not found
packages/cli/src/tools/__tests__/npm-test.ts	Updated test to expect undefined for non-existent versions

Test Plan

# Should fail with error message
node packages/cli/build/bin.js init Test0833 --version "0.83.3" --skip-install
# Error: Unable to find a template for react-native version '0.83.3'...

# Should succeed (if 0.83.2 template exists)
node packages/cli/build/bin.js init Test0832 --version "0.83.2" --skip-install

# Unit tests
yarn test packages/cli/src/tools/__tests__/npm-test.ts