Refactor filter definitions to use shared filter language

[abdulrahmancodes]

Scope & Context

In the current codebase, filter definitions are duplicated across multiple layers:

• GraphQL filter input types in packages/twenty-server/src/engine/api/graphql/workspace-schema-builder/graphql-types/input/
• REST filter parsing logic for string queries like field[eq]:value
• AI Agent tool JSON schema generation in generateFieldFilterJsonSchema function

This duplication leads to inconsistencies, maintenance overhead, and potential bugs when filter logic diverges between layers.

Technical inputs

Create a shared filter language system that can be used across GraphQL, REST, and AI Agent Tool layers.