Remove flash memory store; back tests with session store

[Copilot]

Flash message stores should be implemented against the session backend the user is already using — a generic in-memory store is the wrong abstraction here.

Changes

• Deleted flash/memory-store.lisp and io.github.cl-sdk.wst.flash.memory-store.asd
• Rewrote t/flash-tests.lisp to drive core engine tests (type validation, length limits, queue overflow, session isolation) through a SQLite-backed session-flash-store, matching the pattern already established in flash-session-tests.lisp
• Updated io.github.cl-sdk.wst.flash.test.asd dependencies: replaced flash.memory-store with sqlite, flash.session-store, session, and session.sqlite

Tests that validate engine-level constraints (unknown type, oversized text) fire before any store interaction, so they work without a pre-existing session. Tests that enqueue messages create a real SQLite session first via a make-test-session helper.

(with-flash-engine (session-store engine :max-messages-per-session 1)
  (let ((session-id (make-test-session session-store "s1")))
    (add-flash-message engine session-id :info "one")
    (signals error
      (add-flash-message engine session-id :info "two"))))