feat(indexer): add GET /collections/:address/stats endpoint and tests…

[Sensei-Victor]

Closes #565

---

… (#577)

• Add route that aggregates totalVolume (sum of Sold listings), floorPrice (min price of Active listings), and totalItems (count) for a collection
• Route validates the collection exists first, returns 404 if not found
• Route placed before /collections/:address to prevent param shadowing
• Add collection.findUnique mock to the test mock setup
• Add 4 test cases: happy path aggregation, null floor/volume, 404, 500

Description

Resolves Issue: ### Soroban Safety Checklist

• TTL Extensions: If I modified Persistent storage, I explicitly called extend_ttl for those exact keys immediately after setting them.
• Instance TTL: I ensured extend_instance_ttl is called if this is a public, state-modifying entry point.
• Authorization: I verified that require_auth is used correctly and doesn't accidentally block approved operators or token owners.
• Gas Efficiency: I have avoided putting storage reads/writes or .get(i).unwrap() host calls inside loops.
• State Bloat: I have not used unbounded Vec arrays for global state tracking.
• Signature Security: If I implemented off-chain signatures, the digest explicitly includes the contract address to prevent replays.
• Front-Running Protection: If I used deploy_v2, I hashed the caller's address into the deployment salt.
• Error Handling: I avoided using .unwrap_or() combined with .saturating_sub() to silently hide balance underflows.

Testing

• I have added unit tests to cover my changes and tested edge cases.
• All tests pass locally (cargo test).

Closes #577

Additional Context

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[Ultra-Tech-code]

@Sensei-Victor, Some of the CI checks are failing. Kindly investigate and fix the issue. Thank you your contribution

[Sensei-Victor]

Could someone please approve the pending workflow so the checks can run? Thank you.