fix(@angular/build): remove default for unit-test coverage option

Removing the default value of `false` for the `coverage` schema option in the
unit-test builder. When a default was present, it forced the coverage check in
the Vitest plugins to override any user-provided `coverage: { enabled: true }`
values set within a custom `vitest.config.ts`.

Removing the default explicitly allows the option to be `undefined`, respecting
the fallback behavior of the runner configuration.

The description was also updated to explicitly document how the Vitest runner
resolves the coverage configuration when the builder flag is omitted.

Author: clydin

packages/angular/build/src/builders/unit-test/schema.json

@@ -75,8 +75,7 @@
     },
     "coverage": {
       "type": "boolean",
-      "description": "Enables coverage reporting for tests.",
-      "default": false
+      "description": "Enables coverage reporting for tests. If not specified, the coverage configuration from a runner configuration file will be used if present. Otherwise, coverage is disabled by default."
     },
     "coverageInclude": {
       "type": "array",