fix: surface Python errors during agentcore dev

[notgitika]

Description

Surface Python process errors during agentcore dev instead of showing generic messages like "Failed: fetch failed" or "Internal Server Error".

• Detect Python tracebacks in stderr and show only user-code frames + exception (filter out internal site-packages/stdlib frames)
• Show Python errors inline in the conversation (red) with a "See logs for full stack trace" hint (cyan)
• Throw on HTTP 500 responses instead of displaying raw body as a normal response
• Buffer recent stderr and re-emit as errors on non-zero process exit
• Improve exit message to "Server crashed (code N)" for non-zero exits

Related Issue

Closes #257

Documentation PR

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

Checklist

Yes

• 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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