docs(release-notes): v0.186.2

[sg-doc-holiday]

This PR was triggered by the publication: gOuroboros-Release-Notes

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Release notes

• Adds detailed release notes for v0.186.2, including summaries of clarifications, bug fixes, and improvements for tagged set decoding and MultiAsset handling
• Documents negative test fixtures for duplicate member validation and explains updated error handling
• Updates prior v0.186.1 wording for clarity and consistency
• Ensures that the full historical release sequence with categorized changes is maintained in RELEASE_NOTES.md
• Refines and condenses the v0.186.2 summary in RELEASE_NOTES.md for greater clarity and more direct emphasis on duplicate-member validation and MultiAsset decoding changes
• Iteratively updates the v0.186.2 release notes wording for stronger and clearer emphasis on duplicate-member validation and MultiAsset map key handling
• Refines the prior v0.186.1 release notes for clarity and consistency

This covers 2 commits.

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