fix(ts): disable tree shaking for aws sdk v3 when not external due to rolldown bug

[cogwirrel]

Reason for this change

Rolldown tree-shakes the AWS SDK too aggressively and removes required code. See the following issue for details: rolldown/rolldown#6513

Description of changes

This change ensures that any bundles targeting lambda are generated with the AWS SDK as external as it's provided by the lambda runtime (ts#smithy-api and ts#trpc-api were missing this - ts#lambda-function already had this set).

For projects which don't target lambda and don't externalise the AWS SDK (only ts#mcp-server right now which runs on agentcore runtime), we disable tree shaking for AWS SDK dependencies via a custom plugin.

Additionally set platform to node by default in rolldown config - when omitted rolldown defaults to browser but we're only using it for bundles targeting the node runtime.

Description of how you validated changes

Unit tests. Deployed a ts#mcp-server which uses the dynamodb client.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.27%. Comparing base (8486855) to head (a804939).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #327      +/-   ##
==========================================
+ Coverage   96.25%   96.27%   +0.02%     
==========================================
  Files          91       91              
  Lines        9002     9052      +50     
  Branches     1084     1092       +8     
==========================================
+ Hits         8665     8715      +50     
  Misses        327      327              
  Partials       10       10              

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