Call build:prepare hook on rebuild in watch mode #972

Clear and concise description of the problem

When running in watch mode, tsdown currently only calls the build:done hook on each rebuild. The build:prepare hook is never invoked after the initial build.
This makes it difficult to adopt tsdown in projects that require code generation before building — for example, those using graphql-codegen. Since build:prepare is the natural place to trigger such pre-build steps, skipping it on rebuilds means generated code becomes stale during development.

Suggested solution

Invoke the build:prepare hook at the start of each rebuild cycle in watch mode, just as it is called before the initial build.

Alternative

Expose a dedicated watch-specific hook (e.g. watch:rebuild) that fires before each rebuild, so users can run pre-build tasks without relying on build:prepare semantics.

Additional context

Tools like graphql-codegen need to run before TypeScript compilation to produce up-to-date type definitions. Without build:prepare being called on each rebuild, developers lose the hot-reload experience that watch mode is meant to provide, and must resort to running a separate watcher process in parallel.

Validations

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• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.