feat: Implement tooltip on custom call type

[lornakelly]

Description

Spike #95

Add a small, reusable tooltip component and use it to replace the native browser title tooltip currently shown on custom call type badges.

Motivation

• The native title tooltip is inconsistent with the editor's design system, can't be styled for dark mode so needs to be replaced
• Custom call badge text can be truncated, a proper tooltip is the mechanism for revealing the full custom function name on hover
• Establishes a single reusable tooltip building block the rest of the editor can adopt

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.