feat: inline validation markers in the text edit #227

Description

Surface existing validation errors as inline markers in the text editor (squiggles/gutter markers), alongside the on-canvas error surfacing we already have.

Motivation

Errors shown only on the canvas force users to hunt for the offending line themselves - inline markers put the error where the fix happens, which is the feedback loop developers expect from any code editor

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.