Fix threshold counting for duplicate public keys

[rdimitrov]

Summary

• Count verified threshold contributions by resolved public key fingerprint instead of key ID.
• Prevent duplicate key records for the same public key from satisfying multi-key thresholds.
• Add regression coverage for ECDSA, Ed25519, and RSA duplicate-public-key cases.

Testing

• go test ./metadata -run 'TestVerifyDelegate(Duplicate|Threshold|$)' -count=1 -v

[Copilot]

Pull request overview

This PR fixes delegation threshold verification so duplicate metadata key records that resolve to the same public key cannot satisfy multi-key thresholds by themselves.

Changes:

• Counts verified threshold contributions by SHA-256 fingerprint of the resolved public key.
• Adds regression tests for duplicate public keys across ECDSA, Ed25519, and RSA cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
metadata/metadata.go	Uses PKIX public-key fingerprints instead of key IDs when counting verified threshold contributors.
metadata/metadata_test.go	Adds duplicate-public-key regression coverage for delegated target verification.

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[kommendorkapten]

Thanks, looks good.