fix(training-agent): scope affected packages

[sangilish]

Summary

• Track package ids actually mutated by update_media_buy package updates and new_packages.
• Return only those packages in affected_packages while preserving the full package snapshot in packages.
• Add a multi-package inline-creative regression test covering the Clarify inline creative management #5365 follow-up review note.

Tests

• npx prettier --check /tmp/adcp-minimal-5365/server/src/training-agent/task-handlers.ts /tmp/adcp-minimal-5365/server/tests/unit/training-agent.test.ts /tmp/adcp-minimal-5365/.changeset/affected-packages-mutated-only.md
• node --check /tmp/adcp-minimal-5365/server/src/training-agent/task-handlers.ts

Follow-up for #5365.

[aao-release-bot]

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