Remove MailboxService B3 AiConfig defensive read

Context

This is a narrow B3 cleanup child under #12461 / #12456 for the Memory Core mailbox reply-policy read.

Current verified source on origin/dev:

const strictReplyPolicy = aiConfig.mailbox?.defaultReplyPolicy === 'blocked';

V-B-A collision sweep run before filing:

• git grep -n -E "(aiConfig|AiConfig)(\.[A-Za-z0-9_]+)*\?\." origin/dev -- ai/services/memory-core/MailboxService.mjs found only the MailboxService reply-policy read for this scope.
• gh issue list --state open --search "MailboxService defaultReplyPolicy AiConfig B3" returned no open issues.
• gh pr list --state open --search "MailboxService defaultReplyPolicy AiConfig" returned no open PRs.
• Existing B3 lanes are disjoint: maintenance scripts (#12541/#12542), GitLab HealthService (#12526/#12528), and ConceptDiscoveryService (#12538/#12539).

Problem

MailboxService.addMessage() gates strict reply behavior using an optional-chain read of aiConfig.mailbox.defaultReplyPolicy. Under ADR 0019, resolved AiConfig leaves are the use-site source of truth and B3 defensive reads should fail loudly instead of silently degrading when a required config subtree is absent.

The current ?. masks missing mailbox configuration by treating the strict policy as false, which can weaken the configured reply-policy contract without surfacing the configuration drift.

Architectural Reality

ai/mcp/server/memory-core/config.mjs defines the mailbox subtree and defaultReplyPolicy leaf. MailboxService is a consumer of that resolved config, not an owner or fallback surface.

B3 cleanup means removing defensive optional chaining on config reads while preserving the existing policy behavior:

• 'blocked' enables strict reply-policy enforcement.
• 'open' keeps reply behavior permissive.
• Missing config should fail at the read site rather than becoming an implicit open policy.

Fix

Replace the defensive read in ai/services/memory-core/MailboxService.mjs with a direct resolved-leaf read:

const strictReplyPolicy = aiConfig.mailbox.defaultReplyPolicy === 'blocked';

Do not introduce a local fallback, alias, re-export, or default in MailboxService.

Contract Ledger

Decision Record Impact

Aligned with ADR 0019: the reactive Provider-backed AiConfig singleton remains the SSOT. This ticket removes a consumer-side defensive read and does not create new config ownership.

Acceptance Criteria

• MailboxService.addMessage() no longer uses aiConfig.mailbox?.defaultReplyPolicy.
• The replacement is a direct resolved-leaf read with no fallback.
• Focused MailboxService unit coverage still passes for blocked and open reply-policy behavior.
• The change does not alter mailbox policy semantics, A2A message delivery, or permission behavior.

Out of Scope

• B4 cleanup of test-level singleton mutation patterns.
• Changing the default reply policy.
• Other #12461 B3 clusters in TransportService, MemoryService, SummaryService, SessionService, CollectionProxy, Orchestrator, BaseServer, or Logger.

Related

• Parent: #12461
• Epic: #12456
• ADR: learn/agentos/decisions/0019-aiconfig-reactive-provider-ssot.md

Origin Session

dcdaac0b-9ae0-45b5-b4da-da39541af497

Retrieval Hint

MailboxService defaultReplyPolicy AiConfig B3 ADR 0019