fix: tighten API client response types

[deepsikha-dash]

Description

Tightens frontend API client response typing by replacing any return values with explicit response interfaces and typed request contracts.

Changes include:

• Added explicit response types for settings and task status endpoints.
• Typed notification rules and notification history responses.
• Added raw workflow and workflow execution response types.
• Updated task result typing to reflect nullable responses.
• Updated the task subscription test resolver type to match the typed getTaskStatus response.

Related Issues

Closes #1406

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Ran npm run typecheck successfully.
• Ran targeted frontend tests successfully: 21 tests passed.
• Ran the full frontend test suite successfully: 530 tests passed across 59 test files.
• Ran npm run build successfully.
• Ran git diff --check successfully.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.