Add drag upload language tab regression test

[saurabhhhcodes]

Summary

• Add Playwright regression coverage for issue [Bug] Drag-and-drop file uploads do not auto-select the detected language tab #308.
• Verify a dropped .ts file updates the editor and auto-selects the TypeScript language tab after starting from another tab.

Validation

• npm install
• node /private/tmp/verify-ai-dev-assistant-drop-upload.js
• npm run test:e2e:ci was attempted, but the configured backend web server could not start in this environment because the config invokes python and then /usr/bin/python3 is Python 3.9, which does not support the backend's str | None type syntax.

GSSoC

• Addresses gssoc2026
• Labels: type:bug, type:frontend, good first issue, level:beginner

Closes #308

[saurabhhhcodes]

Updated this branch with the latest main; the refreshed run is green now.

Passing checks on the updated head:

• Frontend Check
• Lint
• test (3.11)
• test (3.12)
• frontend static checks
• PR Analysis

Ready for maintainer review/merge from my side.

[imDarshanGK]

@saurabhhhcodes
update your branch with the latest main changes

[saurabhhhcodes]

Updated this branch with the latest main again after your request. The refreshed checks are green now:\n\n- Frontend Check\n- Lint\n- analyze\n- frontend static checks\n- test (3.11)\n- test (3.12)\n\nReady for review/merge from my side.

[saurabhhhcodes]

Updated this branch with latest upstream main again and pushed one small test compatibility fix after the merge. The frontend sample-comment static check now works with the new ESM test package metadata that landed upstream.

Current GitHub checks on the refreshed head are all green:

• Frontend Check
• Lint
• analyze
• frontend static checks
• test 3.11
• test 3.12

Local validation also passed for the frontend static check, frontend security tests, Ruff backend lint, and git diff whitespace check. The branch should be ready for review/merge from my side.

[saurabhhhcodes]

Refreshed this branch against latest main one more time after new upstream commits landed.

Current GitHub checks are green again on the new head:

• Frontend Check
• Lint
• analyze
• frontend static checks
• test 3.11
• test 3.12

Local validation also passed for frontend static check, frontend security tests, Ruff backend lint, py_compile for the newly merged VS Code package test using a temp pycache, and git diff whitespace check. Ready for review/merge from my side.

[saurabhhhcodes]

Current head is green and this closes #308. If accepted, could you please mirror the issue scoring labels on the PR before merge: gssoc2026, level:beginner, type:bug, type:frontend, plus gssoc:approved / quality label if those are used for final scoring?