refactor(valkey): ambientMesh becomes an object carrying Istio's two constants

Gitea #143, maintainer's instruction. The HBONE port and the redirected probe
source are ISTIO CONSTANTS, not standards, so they move from template literals
into values with documented defaults - an operator can then correct them when
Istio changes a number, without waiting for a chart release.

One object under one "enabled", so the two halves still cannot be set
separately; two values could drift apart, which is what this design exists to
prevent. Every read is null-safe (#99), so a deleted object or one carrying
only "enabled: true" falls back to the defaults declared here rather than
dereferencing nil - the defect #136 shipped one value away.

Provenance now written on each constant: 15008 is Istio's HBONE port, not
IANA-registered for this and unknown to Kubernetes; 169.254.7.127 is link-local
(RFC 3927, never routed), its IPv6 counterpart carries the same digits plus
1337, and neither the NetworkPolicy API nor the kubelet knows the address -
only Istio's data plane rewrites the probe source onto it. Both read for
ztunnel 1.30.6, and a change in either fails silently.

The exception in ci/run-networkpolicy-test.sh stays pinned to the numbers that
test names, and additionally asserts this chart declares the same defaults -
never "whatever the value says", which would let a mistyped port in through a
configurable field.

No version change: nothing is published yet, the default is still off, and the
render at defaults is byte-identical - re-diffed against origin/main.

No redirectedProbeSources here, in neither values.yaml nor the schema: this
chart offers no networkPolicy.probeSources, so its probe rule is never
source-restricted and no ipBlock is ever rendered. The key would reach no rule
and read as a control that does nothing, which is worse than one that is
absent. ci/run-networkpolicy-test.sh asserts that absence rather than
tolerating it.

valkey/values.yaml

@@ -313,5 +313,34 @@ networkPolicy:
     # NOT PROVEN: that 15008 alone is enough comes from Istio's documentation;
     # measured on this cluster is only which listeners ztunnel opens, and no
     # namespace carries istio.io/dataplane-mode yet.
-    ambientMesh: false
+    # BOTH NUMBERS BELOW ARE ISTIO CONSTANTS, NOT STANDARDS, and they are values
+    # rather than template literals for exactly that reason: a vendor constant
+    # can change, and an operator should be able to correct it without waiting
+    # for a chart release. Their provenance is written on each one.
+    #
+    # DELETING THIS OBJECT IS SAFE. Every read falls back to the defaults
+    # declared here (#99), so "ambientMesh: null", or an object carrying only
+    # "enabled: true", renders the documented defaults instead of failing.
+    ambientMesh:
+      # The switch. Everything else here is inert while it is false, and the
+      # chart renders exactly as it did before the object existed.
+      enabled: false
+
+      # ISTIO'S HBONE PORT. A constant Istio chose, not a standard: not
+      # registered with IANA for this, prescribed by no protocol, and unknown to
+      # Kubernetes. Read for ztunnel 1.30.6 (measured on this cluster: ztunnel
+      # opens [::]:15008 as its inbound listener).
+      #
+      # If Istio changes it, a pod in the mesh cannot be reached. Change it here
+      # in all 30 charts AND in the pinned expectation inside
+      # ci/run-networkpolicy-test.sh - the test is deliberately what forces
+      # someone to look, because the exception it grants this port is pinned to
+      # the number IT knows and not to whatever this value says. A value-driven
+      # exception would let a mistyped port through, which is the hole design A
+      # of #143 was rejected for.
+      #
+      # NOT MEASURED: that this port ALONE suffices comes from Istio's
+      # documentation. Measured here is only which listeners ztunnel opens, and
+      # no namespace carries istio.io/dataplane-mode yet.
+      port: 15008