refactor(constants): create error_constants.py — centralize repeated error strings (#3530)

[mrveiss]

Summary

Creates autobot-backend/constants/error_constants.py and replaces the highest-density repeated error message strings across 6 API files.

New file: constants/error_constants.py

Defines named constants for the four main error families: ERR_*_NOT_FOUND, ERR_INVALID_CREDENTIALS, ERR_INVALID_TOKEN, ERR_FAILED_TO.

Files updated

File	Occurrences replaced
api/security_assessment.py	11 × "Assessment not found" → ERR_ASSESSMENT_NOT_FOUND
api/terminal.py	8 × "Session not found" → ERR_SESSION_NOT_FOUND
api/files.py	9 × file/directory not-found variants → ERR_FILE_NOT_FOUND, ERR_DIRECTORY_NOT_FOUND, etc.
api/knowledge_connectors.py	6 × "Connector not found" → ERR_CONNECTOR_NOT_FOUND
api/workflow.py	5 × "Workflow not found" → ERR_WORKFLOW_NOT_FOUND
api/auth.py	"Invalid username or password" + "Invalid token" → named constants
constants/__init__.py	Exports all new constants

Skipped (per issue guidance)

• api/memory.py — all "Failed to X" are f-strings with unique operation names; no identical repeats qualify
• api/batch_jobs.py — "Job/Template {id} not found" are f-strings with dynamic IDs

Closes #3530

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!