feat(gateway): migrate GatewayHost.ResolveSessionType off SessionId.IsSubAgent substring (Phase 5 / F-6 step 2)

[sytone]

Summary

Phase 5 / F-6 step 2: migrate GatewayHost.ResolveSessionType off the legacy SessionId.IsSubAgent substring check. The sub-agent session-type discriminator is now driven by the typed AgentDescriptor.Kind signal sourced from IAgentRegistry. Closes #554.

This is the second migration step after PR #556 (which introduced AgentKind). The substring path stays alive only in the architecturally-allowlisted call sites: SessionStoreBase.InferSessionType (legacy read path), InProcessIsolationStrategy (defense-in-depth OR gate), and SessionsController (deferred to #555).

Changes

File	Change
src/gateway/BotNexus.Gateway/GatewayHost.cs	ResolveSessionType is now an instance method that reads _registry?.Get(session.AgentId)?.Kind instead of session.SessionId.IsSubAgent. New optional IAgentRegistry? registry = null ctor parameter (preserves back-compat with the 7 test composition roots). Cached CronChannelKey static field to remove per-dispatch allocation. New isNewSession parameter to enable the persisted-AgentSubAgent preservation rule (see Critique below).
tests/architecture/BotNexus.Architecture.Tests/AgentKindArchitectureTests.cs	Removed GatewayHost.cs allowlist entry. Rewrote XML doc remarks to record migration history and call out SessionsController (#555) as the remaining deferred site.
tests/gateway/BotNexus.Gateway.Tests/GatewayHostResolveSessionTypeTests.cs	NEW behavioural test file. 9 tests pin every row of the migration truth table including the persisted-AgentSubAgent preservation regression.

Critique sweep — 3 background agents

Following the standing multi-model critique policy. All three completed before commit-amendment.

Reviewer	Model	Verdict	Adopted
security-review	gpt-5.5	✅ PASS — no vulnerabilities (7 focus areas: privilege escalation, registry poisoning, TOCTOU, info disclosure, optional DI failure mode, behaviour drift vs #556, test surface)	n/a
code-review plan-vs-impl	claude-opus-4.7 (after 4.6 returned 401)	✅ DoD #1 & #2 PASS · ⚠️ 3 SHOULD-CONSIDERs	2 of 3 adopted: added missing baseline truth-table row, renamed fragile literal. Skipped: dropping the "no registry injected" test (subtly different DI scenario from "null returned from registry")
rubber-duck bug-hunt	gpt-5.3-codex	🔴 1 HIGH · 🟡 1 MEDIUM · 🟢 1 LOW	All 3 adopted

HIGH adopted (production-bug-prevention)

Persisted AgentSubAgent sessions could be silently downgraded to UserAgent on next dispatch when the sub-agent descriptor had been Unregister()'d (sub-agent completion via DefaultSubAgentManager.cs:557) or was transiently absent after a gateway restart. Downstream effects include flipping Session.IsInteractive (Session.cs:35), SessionWarmupService.cs:183-190 visibility, and PreCompactionMemoryFlusher.cs:33-36 / SessionEndMemoryFlusher.cs:34-36 gating.

Fix: threaded isNewSession into ResolveSessionType and added a fail-closed branch that preserves AgentSubAgent on EXISTING sessions when no typed signal contradicts. The branch deliberately requires isNewSession=false so fresh dispatches do NOT re-open a substring back door — the #554 architecture-fence invariant stays intact.

Regression test: DispatchAsync_WhenSessionAlreadyPersistedAsAgentSubAgent_AndRegistryHasNoDescriptor_PreservesAgentSubAgent seeds an existing AgentSubAgent session via GetOrCreateAsync, then re-dispatches with a null registry and asserts the type is preserved.

LOW adopted

ChannelKey.From("cron") allocated per dispatch → cached as private static readonly CronChannelKey.

Plan-vs-impl adopted

• Added baseline truth-table row: DispatchAsync_WhenRegistryReturnsNamedDescriptor_AndSessionIdIsPlain_StampsAsUserAgent.
• Renamed fragile literal "session-with-subagent-descriptor" → "plain-session-id-1" so the SubAgent classification can only come from the typed descriptor, not from any accidental substring match.

Truth table (pinned by tests)

Descriptor.Kind	SessionId shape	Channel	isNewSession	Expected SessionType	Test
SubAgent	plain	web	true	AgentSubAgent	…ReturnsSubAgentDescriptor_StampsSessionTypeAsAgentSubAgent
Named	::subagent:: infix	web	true	UserAgent (typed signal trumps substring)	…ReturnsNamedDescriptor_AndSessionIdHasSubAgentInfix_StampsAsUserAgent ⭐ critical pin
null (registry returns null)	::subagent:: infix	web	true	UserAgent (no substring fallback)	…RegistryHasNoDescriptor_AndSessionIdHasSubAgentInfix_StampsAsUserAgent
null (no registry injected)	::subagent:: infix	web	true	UserAgent (no substring fallback)	…RegistryNotProvided_AndSessionIdHasSubAgentInfix_StampsAsUserAgent
null (deregister race)	any	any	false + persisted=AgentSubAgent	AgentSubAgent (preserved)	…SessionAlreadyPersistedAsAgentSubAgent_…_PreservesAgentSubAgent ⭐ regression pin
Named	plain	web	true	UserAgent (baseline)	…ReturnsNamedDescriptor_AndSessionIdIsPlain_StampsAsUserAgent
Named	Soul shape	web	true	Soul	…WhenSessionIdIsSoul_StampsAsSoul
Named	plain	cron	true	Cron	…WhenChannelTypeIsCron_StampsAsCron
SubAgent	plain	cron	true	AgentSubAgent (precedence)	…WhenChannelTypeIsCron_AndDescriptorKindIsSubAgent_PrefersSubAgentBucketing

Validation

• Build: 0 warnings, 0 errors under TreatWarningsAsErrors.
• GatewayHostResolveSessionTypeTests: 9/9 passed (7 original + 2 added during critique fold-in).
• BotNexus.Gateway.Tests: 1815/1816 passed (1 skipped, 0 failed).
• Architecture fence (AgentKindArchitectureTests): 49/49 passed.
• Full solution suite: ~4,600 passed / 41 skipped / 0 failed.

Follow-up

• [Gateway] Migrate SessionsController off SessionId.IsSubAgent (Phase 5 follow-up) #555 — SessionsController substring filter migration. Bigger scope (~200–400 LoC; requires persisting a SessionType discriminator on the session list query). Will be the next PR in the Phase 5 / F-6 sequence.

Closes #554