fix: agentcore dev not working in windows

[avi-alpert]

Description

On Windows, agentcore dev fails with Error: spawn start ENOENT when trying to open the browser. This is because start is a cmd.exe built-in, not a standalone executable, so spawn('start', [url]) can't find it.

The fix spawns cmd directly with /c start as arguments (spawn('cmd', ['/c', 'start', url])). Also extracts the browser-opening logic into a reusable openBrowser() utility in utils.ts with tests for all three platforms (macOS, Windows, Linux).

Related Issue

Closes #945

Documentation PR

N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

How have you tested the change?

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots
• Manually tested agentcore dev on a Windows machine and verified the browser opens without errors

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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