chore: Make StorableAnchor a structural super-type of StorableFixedAnchor

[aterga]

Motivation

This PR proposes a new storage model for anchors, extending the existing StorableAnchor type with fields to cover all data stored in StorableFixedAnchor. The idea is to eventually migrate all anchor data to StorableAnchor and obsolete StorableFixedAnchor, which limits the data per anchor to 4 KB.

Changes

• Add optional fields to StableAnchor to represent data that is currently stored in StableFixedAnchor.
• Extend From<Anchor> for (StableAnchor, StableFixedAnchor) to populate the new fields.

Tests

13 unit tests in from_conversion_tests module covering From<Anchor> for (StorableFixedAnchor, StorableAnchor) conversion:

Passkey Credentials

• should_convert_passkey_credentials_with_authentication_purpose - Authentication passkeys → StorablePasskeyCredential with field mapping
• should_convert_passkey_credentials_with_recovery_purpose - Recovery passkeys → added to passkey list at the end
• should_convert_mixed_device_types - Mixed device types → proper ordering and separation

Recovery Keys

• should_convert_recovery_keys_from_seed_phrase - Seed phrases → StorableRecoveryKey (excluded from passkey list)
• should_convert_protected_recovery_key - Protected recovery keys → is_protected: Some(true)

Edge Cases

• should_handle_empty_alias - Empty aliases → None instead of Some("")
• should_use_default_origin_for_passkeys_without_origin - Missing origin → defaults to "https://identity.ic0.app"
• should_handle_devices_without_credential_id - Devices without credential_id → excluded from both lists

Preservation

• should_preserve_fixed_anchor_fields - StorableFixedAnchor → preserves devices, metadata, created_at
• should_set_name_and_created_at_in_storable_anchor - StorableAnchor → name and created_at_ns set correctly
• should_convert_openid_credentials - OpenID credentials → preserved during conversion

Empty State

• should_handle_anchor_with_no_devices - Empty anchor → empty vectors

[Copilot]

Pull request overview

This PR extends the StorableAnchor type to become a structural super-type of StorableFixedAnchor by adding fields that cover all anchor data. This is part of a migration strategy to eventually obsolete StorableFixedAnchor, which has a 4 KB data limit per anchor.

Key changes:

• Added new structs StorablePasskeyCredential and StorableRecoveryKey to represent device credentials in the extended storage model
• Extended StorableAnchor with new fields: created_at_ns, passkey_credentials, and recovery_phrases
• Updated the From<Anchor> implementation to populate the new fields by converting device data into the new storage structures

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/internet_identity/src/storage/storable/anchor.rs	Defines two new structs (StorablePasskeyCredential, StorableRecoveryKey) and adds three new fields to StorableAnchor
src/internet_identity/src/storage/anchor.rs	Updates conversion logic to populate the new StorableAnchor fields from Anchor devices
src/internet_identity/src/storage.rs	Fixes typo in error message from "StableAnchor" to "StorableAnchor"
src/internet_identity/src/main.rs	Updates comment with a spelling correction

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

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[sea-snake]

LGTM

As discussed, special edge case (purpose auth without credential id) will be handled differently in follow up PR.