[explorer/puller] fix: recover Symbol metadata state during rollback

[daoka]

What is the current behavior?

Symbol Metadata rollback recovery derives natural keys only from current
symbol_metadata rows updated at or after the fork height.

Mosaic Metadata transactions preserve the unresolved target mosaic ID in their
body, but do not persist the resolved target ID needed to reconstruct the
canonical Metadata natural key.

This is a follow-up to #2463.

What's the issue?

An orphaned Metadata update can leave an empty value. Symbol core removes the current Metadata entry in that case.

If the Explorer current-state row has already been removed, rollback recovery has no natural key with which to query the canonical Node state. The missing Metadata entry therefore cannot be restored from current rows alone.

Mosaic aliases require additional handling because the raw transaction body contains the unresolved mosaic ID, while the Metadata natural key uses the resolved mosaic ID.

How have you changed the behavior?

This change persists the resolved Mosaic Metadata target as a private
symbol_transaction_mosaics relation:

• role = 'metadata_target'
• amount = 0
• position = 0
• exactly one relation for each Mosaic Metadata transaction

Account and Namespace Metadata transactions do not create this relation. The unresolved values in body and raw_payload remain unchanged.

Rollback recovery now:

1. Collects Metadata keys from current-state rows.
2. Reconstructs Metadata keys from confirmed transactions at or after the fork height.
3. Uses the private resolved relation for Mosaic Metadata instead of the raw body.targetMosaicId.
4. Canonicalizes scoped Metadata keys and non-null target IDs to uppercase 16-character hexadecimal strings.
5. Deduplicates both key sources using the complete Metadata natural key.
6. Fetches all canonical state before invoking the existing atomic rollback repair.

Malformed transaction bodies, missing or duplicate target relations, invalid sentinels, and malformed hexadecimal identifiers fail deterministically before rollback writes begin.

The exactly-one relation contract is enforced by application validation rather than a partial unique index. The existing relation primary key permits different mosaic IDs for the same transaction and role, and retaining application validation allows malformed persisted state to be detected and tested explicitly.

metadata_target is private persistence state. It must not be exposed through public transferMosaicId searches or Mosaic-detail transaction results.

There is no public API breaking change. This updates the pre-release declarative schema without adding an ALTER TYPE compatibility shim. Existing development databases with the old enum or non-canonical Metadata keys must be recreated.

REST, frontend, NEM, and the merged Lock synchronization behavior are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.36%. Comparing base (d73599a) to head (2293d4a).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2465      +/-   ##
==========================================
- Coverage   95.63%   95.36%   -0.28%     
==========================================
  Files         719      679      -40     
  Lines       53893    50630    -3263     
  Branches     1405     1405              
==========================================
- Hits        51540    48282    -3258     
+ Misses       2295     2290       -5     
  Partials       58       58              

Flag	Coverage Δ
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wallet-mobile-symbol	94.18% <ø> (ø)

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