test: wait for app readiness before late session injection (#4224)

Root cause:
The API socket is bound before App evaluates session restore. Waiting only
for that socket lets the late-session test inject its file during startup,
so the server may load it normally instead of exercising late-file recovery.
The missing backup then fails an otherwise correct persistence assertion.

Fix:
Wait for a read-only workspace-list request to pass through App and assert
that the workspace list is still empty before injecting the session file.
Keep the existing autosave deadline and byte-for-byte backup assertion.
Add no sleeps, retries, longer timeouts or production changes.

Validation:
The original assertion fails with unchanged master. With the barrier, five
independent runs pass. Full native just ci passes 3624 Rust tests with six
default skips, formatting, all-target Clippy, 112 maintenance tests, six
architecture tests and 39 integration-asset tests. All seven docs contract
tests pass. Local Windows cross-lint was not run because its SDK is absent.

Co-authored-by: JJ Liebig <jonathan.liebig@gmail.com>

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