Import optional dependencies inside with optional_import_block

[kumaranvpl]

Why are these changes needed?

Related issue number

Closes #1379

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[kumaranvpl]

Previously failing tests cases are passing now in this branch with the fix
https://github.com/ag2ai/ag2/actions/runs/13900538026
https://github.com/ag2ai/ag2/actions/runs/13900540288

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 99 files with indirect coverage changes

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