[Architecture Review][P1] Unified multi-tenant / security / quota entrypoint

[qqeasonchen]

Execution meta — see plan: #5296

• Phase: 4 — boundary decoupling
• Severity: P1
• Depends on: [Architecture Review][P0] Single protocol path (HTTP + CloudEvents) and ingress/egress adapters #5299
• Blocks: (none)

Problem

TLS, mTLS, and rate limiting exist as point capabilities, but there is no unified identity and policy context spanning publish, subscribe, ACK, Connector, and A2A.

Proposed direction

Introduce a unified RequestContext at the ingress layer:

RequestContext
- tenantId
- principal
- roles / scopes
- trace context
- quota key
- source identity

Then uniformly implement:

• topic publish/subscribe ACL;
• tenant namespaces;
• subscription, connection, throughput, and backlog quotas;
• Connector credential isolation;
• A2A agent registration and invocation authorization;
• audit events.

Acceptance criteria

• A single RequestContext flows through publish / subscribe / ACK / Connector / A2A paths.
• ACL and quota enforcement is centralized, not scattered per protocol.
• Audit events are emitted for authorized operations.

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Part of the Architecture Review.