build: configure tsdown to output both ESM and CJS

[lppedd]

Motivation and Context

The SDK v2 needs proper packaging for both ESM and CJS consumers.

tsdown can easily do that, but was not properly configured.
Each module system's output is isolated into its own dist/{module-system} subdir.

Fixes #971

How Has This Been Tested?

• Executed all test cases.
• Packaged tarballs and installed them locally.
• Verified the outputted consumer files properly run, and that they properly bundle SDK declarations.

Breaking Changes

There is no "obvious" breaking change that I can think of.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

No AI-assistant was involved in the changes. If something's broken it's my fault.

[changeset-bot]

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Latest commit: 4a86307

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[lppedd]

You need to bump Node.js/Deno/Bun memory a little bit, otherwise it goes out of memory while building with tsdown.
8 gigs are more than enough.

Nevermind. Fixed.

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commit: 4a86307

[lppedd]

Could a collaborator replay the failed action? That looks like a network issue unrelated to the change.