feat: Add API endpoints for chatflow import and export

[manishrawal95]

This PR introduces new API endpoints to programmatically import and export chatflows, resolving issue #1377. This is a critical feature for enabling MLOps and BizOps workflows, allowing teams to version control their chatflows, automate backups, and migrate agent configurations between different environments.

As the creator of open-source tools for AI agent observability like agent-profiler and retrace, I understand the importance of robust operational tooling for managing AI systems at scale. This API provides a foundational building block for more automated and reliable agent deployment pipelines.

Changes Implemented:

• Added GET /api/v1/chatflows/export/:id to export a specific chatflow as a JSON object.
• Added POST /api/v1/chatflows/import to create a new chatflow from a JSON file, with validation.
• Protected the new endpoints with authentication middleware.
• Included API documentation and unit tests for the new functionality.

[gemini-code-assist]

Code Review

This pull request introduces export and import functionality for chatflows, adding new API endpoints and service-level logic for data validation and ID remapping. It also refactors the chatflows controller by introducing helper functions for common parameter and context validation tasks, reducing code duplication. Additionally, unit tests were added for the new import validation logic. Feedback was provided to correct mismatched error message expectations in the new unit tests, as the current assertions would cause the tests to fail.