What this PR does / why we need it:

Feast's RBAC layer grants a fully trusted intra-communication identity to any request whose
preferred_username (OIDC) or service-account name (Kubernetes) claim matches
INTRA_COMMUNICATION_BASE64. That claim was read from an unverified JWT decode performed
before any signature check or IdP round-trip, and the value it was compared against is hardcoded
in Feast's own Helm chart to the constant base64("intra-server-communication").

Any caller able to reach a Feast server could therefore mint

jwt.encode({"preferred_username": "aW50cmEtc2VydmVyLWNvbW11bmljYXRpb24="}, "", algorithm="none")

and have is_auth_necessary() skip every permission check, for every project on that server. The
advisory's PoC reads all permission policies, reads across projects, and creates and deletes
entities, using a token with an empty signature and no roles.

The fix

Both halves have to change, because either one on its own leaves the identity forgeable:

1. The token is signed. IntraCommAuthClientManager signs with HS256 using the shared
   secret instead of algorithm="none", and both token parsers verify that signature before
   reading any claim. A caller that does not hold the secret cannot produce an acceptable token,
   whatever it writes in the payload.
2. The chart no longer ships the secret. INTRA_COMMUNICATION_BASE64 now comes from a
   Kubernetes Secret named by the new intraCommunicationSecret value, and the variable is
   omitted entirely when that is not configured. Signing alone would not help while the key is a
   constant published in this repository.
3. An unset or empty secret disables the path instead of being compared against.
   get_intra_comm_secret() normalizes "" to None, and is_auth_necessary() now requires a
   configured secret before it will skip permission checks, so a blank username cannot match a
   blank environment variable.

The shared logic lives in a new feast/permissions/auth/intra_comm.py, so the OIDC parser, the
Kubernetes parser and the client agree on how the token is minted and verified.

⚠️ Breaking change

Deployments that relied on the chart's default must now create a Secret and set
intraCommunicationSecret.name. Every Feast release that calls another one (a feature server and
the registry server it reads from, for example) has to point at the same Secret, and the value has
to be private to the installation. Until it is configured, intra-server communication is disabled
rather than silently trusting a well-known value: that fail-closed default is deliberate, since
the alternative is the vulnerability being fixed.

Which issue(s) this PR fixes:

Fixes #6785

Advisory: GHSA-h543-6vgr-fm36

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests

Behaviour, measured before and after

Unit tests

test_oidc_inter_server_comm and test_k8s_inter_server_comm previously drove the intra-server
path with a placeholder token and a monkeypatched jwt.decode. Once the signature is checked they
no longer reach that path at all, so they now build a genuinely signed token and assert on it.
New cases cover what the old ones could not:

• test_oidc_intra_comm_rejects_forged_token / test_k8s_intra_comm_rejects_forged_token —
  unsigned, and signed with another secret
• test_intra_comm_disabled_without_secret — unset and empty
• test_intra_comm_token_verifies_only_with_its_own_secret
• test_intra_comm_client_token_is_accepted_by_oidc_parser — the client and the parser still
  agree end to end

$ pytest sdk/python/tests/unit/permissions/ -q
337 passed

Helm chart

$ helm template t . --set feature_store_yaml_base64=abc | grep INTRA_COMMUNICATION
(no output - intra-server communication disabled)

$ helm template t . --set feature_store_yaml_base64=abc \
    --set intraCommunicationSecret.name=feast-intra-comm | grep -A 4 INTRA_COMMUNICATION
- name: INTRA_COMMUNICATION_BASE64
  valueFrom:
    secretKeyRef:
      name: feast-intra-comm
      key: INTRA_COMMUNICATION_BASE64

$ helm lint . --set feature_store_yaml_base64=abc
1 chart(s) linted, 0 chart(s) failed

ruff check, ruff format --check and mypy pass on every changed file.

Misc

Two design points worth a reviewer's opinion:

• The secret still travels in the claim. The parsers keep comparing the claim to the secret,
  which keeps the change small and leaves is_auth_necessary() semantics untouched, but it means
  an observer of one internal request learns the secret. Moving the claim to a fixed non-secret
  marker and relying on the signature alone would fix that; it needs a different
  is_auth_necessary() check, so I left it out of a security fix that should stay reviewable.
  Happy to add it here or follow up separately.
• Upgrade ordering. A signed token is not accepted by an older server and vice versa, so a
  mixed-version rolling upgrade will see intra-server calls fail until both sides are updated. If
  you would rather have a release that accepts both forms first, say so and I will add a
  transition path.