fix(transform): preserve export keywords on nested namespaces

[privatenumber]

Fixes #359

Removes two problematic code sections from PR #352:

1. Lines 26-28: Unnecessary duplicateExports() call that caused superfluous export name duplication in namespace bodies

2. Lines 596-605: Incorrect removal of export keywords from nested namespaces, which changed their accessibility from public to private

Problem

The previous implementation was removing export keywords from nested namespaces, which:

• Changed nested namespaces from public to private scope
• Broke downstream consumption where nested namespaces were accessed
• Broke API documentation which relied on exported namespaces
• Violated the plugin's philosophy: bundle declarations, not transform semantics

Changes

• Removed unnecessary duplicateExports() invocation in preProcessNamespaceBody()
• Removed the else block in fixModifiers() that stripped export from nested namespaces
• Updated test expectations to preserve export keywords on nested namespaces

[typhonrt]

Just in reviewing the code changes via sight reading this indeed solves the problems mentioned. It's great that you listed a general principle of the plugin being "bundle declarations, not transform semantics". I will try your PR fork tomorrow (10/17/25) if this is not merged before then for final real world confirmation. I'm confident though that this PR / change / fix will get your work to a solid functional state. At that point I'm definitely excited to see a new release of rollup-plugin-dts as this is a large improvement in capabilities for namespace support and allows me to do a major release of my own tooling built on top of this plugin.

[typhonrt]

I have tested this PR in the real world against a complex project and the results are 100% great! Thanks again @privatenumber!

@Swatinem I recommend a merge for this PR and it should finish this round of namespace bundling improvements. This is significant new capabilities, so if possible please consider a new package release; perhaps 6.3.0 semver. Once this is publicly available it will allow me to do a major release of my OSS tooling that uses rollup-plugin-dts, so certainly anticipated.