Make resetFixture wait for a cache current with the content it wrote

The readiness probe accepted any 200 from the note+json endpoint, on the
stated assumption that a 200 proves the metadata cache is populated. It
only proves a cache entry exists. Every suite reuses the same fixture
path, so on all but the first reset Obsidian already holds a cache entry
describing the previous test's content, and the plugin's
waitForFileCache hands back whatever entry exists rather than one
current with the latest write. The probe could therefore be satisfied
entirely from a pre-PUT snapshot, letting a test patch and read back
frontmatter from before its own reset.

Comparing the echoed content against the body we PUT closes that
particular window. It does not close the underlying one: the same
stale-cache read is reachable through the plugin's own write-then-read
path, which is a product-level issue rather than a test-harness one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>