fix(plugin-router): preserve default watch option

[juntak45]

Summary

• preserve watch: true when router({}) is called with an empty options object
• add a focused regression test for the plugin dev handler
• add a patch changeset for @granite-js/plugin-router

Test plan

• yarn workspace @granite-js/plugin-router test src/routerPlugin.spec.ts
• yarn workspace @granite-js/plugin-router test
• yarn workspace @granite-js/plugin-core build
• yarn workspace @granite-js/plugin-router typecheck
• yarn eslint packages/plugin-router/src/routerPlugin.ts packages/plugin-router/src/routerPlugin.spec.ts
• yarn workspace @granite-js/plugin-router build

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[changeset-bot]

🦋 Changeset detected

Latest commit: e1b220c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 28 packages

Name	Type
@granite-js/plugin-router	Patch
babel-preset-granite	Patch
create-granite-app	Patch
@granite-js/blur-view	Patch
@granite-js/brownfield-module	Patch
@granite-js/cli	Patch
@granite-js/cookies	Patch
@granite-js/screen	Patch
@granite-js/image	Patch
@granite-js/jest	Patch
@granite-js/lottie	Patch
@granite-js/mpack	Patch
@granite-js/native	Patch
@granite-js/naver-map	Patch
@granite-js/plugin-core	Patch
@granite-js/plugin-env	Patch
@granite-js/plugin-hermes	Patch
@granite-js/plugin-micro-frontend	Patch
@granite-js/plugin-rozenite	Patch
@granite-js/plugin-sentry	Patch
@granite-js/react-native	Patch
@granite-js/style-utils	Patch
@granite-js/utils	Patch
@granite-js/video	Patch
@granite-js/vitest	Patch
@granite-js/deployment-manager	Patch
@granite-js/forge-cli	Patch
@granite-js/pulumi-aws	Patch

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[jingjing2222]

LGTM