Remove outdated GPU marketplace endpoint and fix staking service logic

- Remove duplicate `/marketplace/gpu/{gpu_id}` endpoint from marketplace_gpu.py
- Remove marketplace_gpu router inclusion from main.py (already included elsewhere)
- Fix staking service staker_count logic to check existing stakes before increment/decrement
- Add minimum stake amount validation (100 AITBC)
- Add proper error handling for stake not found cases
- Fix staking pool update to commit and refresh after modifications
- Update CLI send_transaction to use chain

contracts/artifacts/interfaces/IModularContracts.sol/IGasOptimizer.dbg.json

@@ -0,0 +1,4 @@
+{
+  "_format": "hh-sol-dbg-1",
+  "buildInfo": "../../build-info/9733ae384af13e1bc5099101cffcb80a.json"
+}