Expand unit coverage for transient cache behavior and adapter safety guards

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This PR improves test coverage in areas that were untested despite being core cache-safety and data-isolation paths. It adds focused unit tests around transient cache lifecycle behavior and adapter guardrails.

• Coverage expansion: transient cache lifecycle + isolation

  • Added direct tests for StarTransientCache site-scoped set/get/delete behavior.
  • Added multisite isolation coverage to ensure per-site transient keys do not bleed across blog IDs.
  • Added network-transient coverage to verify shared visibility across blogs and proper deletion semantics.

• Coverage expansion: adapter safety semantics

  • Added test coverage for StarCacheAdapter::add() in WP fallback mode to confirm write-once behavior (no overwrite when key exists).
  • Added guard-path coverage for StarCacheAdapter::flush() to ensure dangerous flush is blocked unless explicitly enabled.

• Test fixture hardening

  • Reset transient global state in setUp() to prevent cross-test contamination for newly covered paths.

$this->assertTrue(StarCacheAdapter::add('adapter_add', 'first', 30, 'group'));
$this->assertFalse(StarCacheAdapter::add('adapter_add', 'second', 30, 'group'));
$this->assertSame('first', StarCacheAdapter::get('adapter_add', 'group'));

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Pull request overview

This PR expands PHPUnit unit coverage around two core safety/isolation areas in StarCache: (1) transient caching behavior across site vs network scopes and (2) adapter guardrails for “write-once” semantics and dangerous flush blocking.

Changes:

• Added unit tests for StarTransientCache set/get/delete behavior, including per-blog isolation and network-transient sharing semantics.
• Added unit tests for StarCacheAdapter::add() (WP fallback, no-overwrite) and StarCacheAdapter::flush() (blocked unless explicitly enabled).
• Hardened test isolation by resetting the in-memory transient shim state in setUp().

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