feat: improve test coverage to 80%

[anton-gor-dev]

Current state

Coverage is sitting at ~59% lines / ~50% functions / ~48% branches. The thresholds were intentionally set low to unblock CI, but the goal is to raise them to 80%/80%/70%.

Uncovered areas (based on coverage report)

• packages/plugin/src/vue/devtools/ — DevTools integration (tree builders, state builder) has no tests
• packages/plugin/src/unplugin/find-composables.ts — edge cases: async, generic, overloaded functions
• packages/core/src/runtime/utils/track-composable.ts — component binding, scope-less warning path
• packages/core/src/utils/is-dev.ts — environment detection branches

Approach

• DevTools builders can be tested by calling them with mock ComposableInstance data and asserting the tree shape
• find-composables needs more fixture-based tests for edge cases
• track-composable needs tests with a real Vue getCurrentInstance mock

Acceptance criteria

• lines threshold raised to 80%
• functions threshold raised to 80%
• branches threshold raised to 70%
• CI passes with new thresholds