fix: avoid NPE when mock rule omits httpStatusCode (#6881)

[juicewcode]

Fixes #6881

Changes

• MockPlugin.java: guard httpStatusCode with
  Optional.ofNullable(...).orElse(200), so a missing/null status code
  defaults to 200 OK and the mock response is returned.
• MockPluginTest.java: new unit test covering
  • null httpStatusCode → 200 with mock body (issue regression),
  • an explicitly specified status code (e.g. 404) being honored,
  • a missing cached handle falling through to the plugin chain.

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[Aias00]

Review: fix: avoid NPE when mock rule omits httpStatusCode (#6881)

Approved.

Verification

• MockPlugin.doExecute now wraps mockHandle.getHttpStatusCode() with Optional.ofNullable(...).orElse(200), so a missing/null status code defaults to 200 OK instead of throwing HttpStatus.valueOf(null) NPE.
• Confirmed MockHandle.getHttpStatusCode() returns Integer (nullable) — so the original HttpStatus.valueOf(...) would indeed NPE on an omitted field; the guard is correct and necessary.
• Confirmed the new test references valid symbols: MockPluginHandler.CACHED_HANDLE (a Supplier<CommonHandleCache<...>>) with cachedHandle / removeHandle exist on the head branch.
• New MockPluginTest covers three scenarios: null httpStatusCode → 200 with body, explicit 404 honored, and missing cached handle falls through to the chain.

Notes

• Clean, well-tested defensive fix.

Thanks!