fix(studio): label omitted worker runtime as custom FE-4314 (#49757)

## Problem

Workers with an omitted runtime are displayed as Unknown, even though an
omitted runtime represents a custom worker image.

## Fix

Display Custom when the runtime is omitted. Preserve friendly labels for
known runtimes and raw values for explicit unrecognized runtimes.

## How to test

- Run `node_modules/.bin/vitest --run
components/interfaces/Workers/Workers.utils.test.ts` from `apps/studio`.
- Open a worker whose API response omits `spec.runtime`.
- Expected result: the runtime badge displays Custom.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Worker runtimes without available metadata are now labeled
**“Custom”** instead of **“Unknown.”**
- Updated the related behavior validation to reflect the corrected
runtime label.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/studio/components/interfaces/Workers/Workers.utils.test.ts

@@ -112,8 +112,8 @@ describe('formatRuntime', () => {
     expect(formatRuntime('rust')).toBe('rust')
   })
 
-  it('reports unknown when the API omits the runtime', () => {
-    expect(formatRuntime(undefined)).toBe('Unknown')
+  it('labels an omitted runtime as custom', () => {
+    expect(formatRuntime(undefined)).toBe('Custom')
   })
 })
 

apps/studio/components/interfaces/Workers/Workers.utils.ts

@@ -42,7 +42,7 @@ export const getRuntimeMeta = (runtime: string | undefined): RuntimeMeta | undef
   runtime === undefined ? undefined : RUNTIMES[runtime]
 
 export const formatRuntime = (runtime: string | undefined): string =>
-  getRuntimeMeta(runtime)?.label ?? runtime ?? 'Unknown'
+  getRuntimeMeta(runtime)?.label ?? runtime ?? 'Custom'
 
 // The API reports size as e.g. "2gb-1vcpu"; render the parts when they parse, the raw value if not.
 export const formatSize = (size: string): string => {