refac: Implement Per Asset Timestamp Tracking

[Olowodarey]

This PR fixes a design issue in the RWA Oracle where a single global last_timestamp was used to validate price updates for all assets.

Because different assets can have independent update frequencies and timelines (e.g. stocks vs crypto, different data providers), a global timestamp caused valid updates to be incorrectly rejected.

This change introduces per-asset timestamp tracking while preserving the global timestamp for SEP-40 compatibility.

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Problem

Previously:

• last_timestamp was stored globally in RWAOracleStorage

• Updating one asset advanced the timestamp for all assets

• This caused:

  • First-time updates on other assets to fail
  • Assets with slower update cadences to be blocked
  • Incorrect enforcement of C-02 timestamp validation

Example:

• Update AAPL at t = 1000
• Update TSLA at t = 500 → incorrectly rejected

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Solution

This PR introduces per-asset timestamp validation:

• Added DataKey::LastTimestamp(Asset) to store timestamps per asset
• Timestamp validation (C-02) now checks the asset’s own last timestamp
• Global last_timestamp is still updated for backward compatibility (SEP-40)
• Assets can update independently without interfering with each other

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Implementation Details

New Storage Key

DataKey::LastTimestamp(Asset)

Updated Logic

• set_asset_price_internal now:

  • Validates timestamps per asset
  • Stores per-asset timestamps in persistent storage
  • Continues updating global last_timestamp for SEP-40 compatibility

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Tests Added

A total of 10 unit tests now cover all required scenarios.

Existing / Core Per-Asset Timestamp Tests

• test_per_asset_timestamps_independent
• test_same_asset_requires_newer_timestamp
• test_same_asset_same_timestamp_rejected
• test_same_asset_newer_timestamp_accepted
• test_global_timestamp_still_updated

Additional Coverage & Compatibility Tests

• test_independent_timestamps_for_different_assets
• test_updating_asset_with_older_timestamp_than_another_asset
• test_per_asset_timestamp_retrieval
• test_global_timestamp_still_updates
• test_backward_compatibility_with_sep40

All existing tests continue to pass.

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Verification

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closes #31

[vercel]

@Olowodarey is attempting to deploy a commit to the fabianalejandrosanchezduran-gmailcom's projects Team on Vercel.

A member of the Team first needs to authorize it.

[aguilar1x]

@Olowodarey

This issue depends on #25, please wait until I finish merging that milestone so you can validate your PR.

[aguilar1x]

@Olowodarey

Pr #25 merge, you can now continue with your issue.

[Olowodarey]

@aguilar1x check now

[aguilar1x]

@aguilar1x check now

Can you fix the conflicts in your branch?