Carry the tenant claim's provenance, and correct a guard the ruling voided

GH-DEC-2026-013 §5 is a finding nobody asked for and ours to implement: tenant is
a bare string, so a consumer cannot tell a zone the directory asserted about the
person from one a registration supplied about the client they came through.
approval-engine exact-matches that string while its contract reads as though it
relies on the first -- the check is sound and the property a reader infers from
it is absent. gate-house named the property and left the mechanism to us.

Every token now carries tenant_source beside tenant: directory, registration or
default. Advertised in claims_supported, and asserted at the token level on both
grants rather than only in the resolution function, since the claim a consumer
reads is the thing under obligation.

Three values where the ruling names two, which is the judgement here. Labelling
an unasserted profile default as directory would reproduce the same defect one
level down -- a consumer reading an assertion the identity layer never made. The
ruling cites GH-DEC-2026-011 §3 on unknown versus absent for the case it
examined; the same rule applies to our own fallback. The agreement case resolves
to directory deliberately: if a registration declares the zone the directory also
assigned, the directory did assert it, and reporting the weaker source would
understate what is known.

Also corrects the guard shipped in 5f516a0. Its failure message offered two ways
out of adding dynamic registration, and condition (b) voids the second: admitting
dynamic registration voids the registration-bound shape that day, whatever state
the adapter is in. The message named an inadmissible resolution in the exact
place someone would read it while making that change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NV9oijZukGyGbRQGGKnK4P

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 713576@bnt-lap001
Assistant-Session: 384c511d-9bce-4cb8-a676-2aef6c0c8df6

src/internal/server/oidc/discovery.go

@@ -78,7 +78,7 @@ func NewDiscoveryHandler(cfg DiscoveryConfig) http.Handler {
 		// follow them.
 		ClaimsSupported: []string{
 			"sub", "iss", "aud", "exp", "iat",
-			"tenant", "principal_type", "roles", "groups", "assurance", "scope",
+			"tenant", "tenant_source", "principal_type", "roles", "groups", "assurance", "scope",
 			"nonce", "preferred_username", "email", "name", "tenant_roles",
 		},
 		SubjectTypesSupported:     []string{"public"},