chore(api): InvalidatingNodeRegistry ensureOpen + negative test + NodeRegistry @since note (J2/J3/J4)

[DemchaAV]

Summary

Three small carry-over items from the v1.6.7 senior review,
bundled into one PR because they share a topic (the I3 auto-
invalidating registry wrapper) and a CHANGELOG section.

J2 — Symmetric closed-session behaviour

InvalidatingNodeRegistry.register now calls ensureOpen()
before super.register(). Previously the two registration entry
points disagreed on what happens after session.close():

Path	Before	After
session.registerNodeDefinition(...)	IllegalStateException	unchanged — IllegalStateException
session.registry().register(...)	silently mutates closed session	IllegalStateException

ensureOpen() inside the constructor's default-definitions setup
is safe because the closed flag is field-initialised to false
before the constructor body runs.

J3 — Negative test

DocumentSessionTest.registryRegisterOnClosedSessionThrowsIllegalStateException
pins the new contract. Pairs with the existing positive cache-
invalidation test from I3 so the senior-bar "positive + negative"
pattern (cf PR-7.3)
is complete.

J4 — NodeRegistry @since note

Class-level Javadoc on NodeRegistry now explicitly calls out
the v1.6.7 non-final relaxation. The class became non-final in
v1.6.7 (Track I3) so DocumentSession could install the auto-
invalidating subclass; the change was already binary-compatible
(japicmp: semver PATCH). The Javadoc just makes the rationale
discoverable without reading the CHANGELOG.

Test plan

• DocumentSessionTest now has 26 tests (25 prev + 1 new
  negative).
• ./mvnw verify -pl . -P japicmp — 1058 tests, 0 failures.
• japicmp vs v1.6.7 baseline: semver PATCH (compatible — no
  public signature changes; ensureOpen() is a behavioural
  strengthening on an existing public method).