fix: correct grammar and typos across multiple files

[Ricardo-M-L]

Summary

• Fix article usage: a agent → an agent, a answer → an answer, a event → an event, a enum → an enum
• Fix grammar: may due to → may be due to
• Fix test method name typo: mesageplatform → messageplatform

Test plan

• Verify all tests pass with renamed test method

🤖 Generated with Claude Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
autogen/agentchat/group/safeguards/enforcer.py	23.56% <ø> (ø)
autogen/events/base_event.py	90.19% <ø> (ø)
autogen/oai/gemini_types.py	50.00% <ø> (ø)

... and 26 files with indirect coverage changes

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[marklysze]

@Ricardo-M-L thanks a lot for this, would love to merge it, can you sign the CLA please.