refactor(backend): consolidate duplicate venice ai client into single service

[AngelAnselmoRS]

Summary

Resolves #1890

Consolidates duplicate Venice AI client implementations into a single canonical service under backend/src/services/venice/.

Changes

• Moved circuitBreaker.ts and errors.ts to backend/src/services/venice/.
• Re-exported client modules in backend/src/services/venice/index.ts.
• Updated import paths across BaseAgent.ts, client.ts, types.ts, and test files.
• Removed obsolete directory backend/src/venice/.
• Added regression test singleClient.test.ts to enforce single service guard.

Verification & Testing

• npm test (Backend/Frontend)
• cargo test (Smart Contracts)
• cargo fmt --check && cargo clippy (Contract Linting)

Contributor Checklist

• PR title follows Conventional Commits (feat:, fix:, docs:, etc.)
• All CI checks pass locally
• Documentation updated in docs/ or README.md if applicable
• No secrets or credentials committed

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[AngelAnselmoRS]

Clarification: This merged PR addresses and resolves issue #491 (Consolidate duplicate Venice AI client implementations into single canonical service).

[AngelAnselmoRS]

Hi @devJaja! GrantFox support mentioned that because the PR was merged before manual assignment, it can't be updated through Discord support. Could you please check on your GrantFox maintainer dashboard if you can trigger the assignment/reward distribution for issue #491? Thank you!