feat: multi-tenant server with path-based routing and task isolation

[zeroasterisk]

Part of #13 (A2A v1.0 Protocol Support)

Adds an optional A2A.Plug.MultiTenant for multi-tenant A2A deployments with path-based routing and per-tenant task isolation.

Changes

New: A2A.Plug.MultiTenant (lib/a2a/plug/multi_tenant.ex)

• Routes /:tenant/:agent/*path to the correct agent process
• Extracts tenant from URL path and injects into conn.assigns (:a2a_tenant, :a2a_agent_name)
• Sets per-tenant base_url for agent cards (e.g., http://host/acme/echo)
• Flows tenant_id into task metadata via A2A.Plug.put_metadata/2
• Supports both static agent maps and A2A.Registry-based dynamic lookup
• Delegates to existing A2A.Plug — no duplication of JSON-RPC handling

Updated: A2A.TaskStore.ETS (lib/a2a/task_store/ets.ex)

• New tenant_ref/2 function creates namespaced store references
• All store operations (get, put, delete, list, list_all) support tenant-scoped refs
• Tenant A's tasks are completely invisible to tenant B
• Plain (non-tenant) refs work exactly as before — full backward compatibility

Tests (24 new, 413 total, 0 failures)

• 11 ETS tenant isolation tests (put/get/delete/list scoping, cross-tenant invisibility, backward compat)
• 13 multi-tenant plug tests (routing, tenant context, per-tenant agent cards, registry lookup, init validation, backward compat)

Design Decisions

• Optional — existing single-tenant A2A.Plug usage is completely unchanged
• Composable — MultiTenant wraps A2A.Plug, doesn't replace it
• Simple — tenant isolation via key namespacing in shared ETS table, no separate tables per tenant

Fixes part of #13