Fix(abstract-utxo,utxo-core): add .js extensions for ESM compatibility

[0x7365746265726E]

Description

ESM builds in @bitgo/abstract-utxo and @bitgo/utxo-core fail in Node.js with ERR_MODULE_NOT_FOUND.

Root cause: tsconfig.esm.json used moduleResolution: "bundler" which doesn't require .js extensions. Node.js ESM requires explicit extensions.

Changes:

• Updated tsconfig.esm.json in both packages: moduleResolution and module → "NodeNext"
• Added .js to all relative imports in src/**/*.ts

Issue Number

Fixes #7660

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

# ESM import
mkdir test && cd test
echo '{"type":"module"}' > package.json
npm install ../BitGoJS/modules/abstract-utxo
npm install ../BitGoJS/modules/utxo-core
node -e "import('@bitgo/abstract-utxo').then(() => console.log(' ESM works'))"
node -e "import('@bitgo/utxo-core').then(() => console.log('ESM works'))"

# CJS import
node -e "require('@bitgo/abstract-utxo'); console.log('CJS works')"
node -e "require('@bitgo/utxo-core'); console.log('CJS works')"

# TypeScript types
echo "import { AbstractUtxoCoin } from '@bitgo/abstract-utxo';" > test.ts
npx tsc --noEmit  # passes

Happy to add a test case or reproduction repo if useful — let me know preferred approach.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My code compiles correctly for both Node and Browser environments
• I have commented my code, particularly in hard-to-understand areas
• My commits follow Conventional Commits and I have properly described any BREAKING CHANGES
• The ticket or github issue was included in the commit message as a reference
• I have made corresponding changes to the documentation and on any new/updated functions and/or methods - jsdoc
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes