LazyPlatformRouter: accept proposal_stores= for parity with PlatformRouter

[bokelley]

Summary

PlatformRouter (the eager router) accepts proposal_stores: Mapping[str, ProposalStore] and exposes proposal_store_for_tenant(tenant_id) — the accessor the framework's proposal_dispatch duck-types via hasattr(platform, "proposal_store_for_tenant"). LazyPlatformRouter doesn't accept the kwarg and doesn't expose the accessor, so adopters wiring LazyPlatformRouter (the multi-tenant lazy-build pattern the docstring recommends for 50+ tenants) can't plug a ProposalStore into the framework's proposal lifecycle.

Where it bites

adcp/decisioning/platform_router.py:

• PlatformRouter.__init__ accepts proposal_stores= (line ~331) and proposal_store_for_tenant is defined (line ~579).
• LazyPlatformRouter.__init__ accepts only proposal_managers= (line ~736); there's no proposal_stores= and no proposal_store_for_tenant method.

adcp/decisioning/proposal_dispatch.py:109:

if hasattr(platform, "proposal_store_for_tenant"):
    store = platform.proposal_store_for_tenant(tenant_id)

→ on LazyPlatformRouter, hasattr returns False, store=None, and maybe_persist_draft_after_get_products / maybe_hydrate_recipes_for_create_media_buy both short-circuit. Adopters get the v1 (no-proposal) path silently.

Adopter workaround (we shipped this)

Subclass LazyPlatformRouter and add the accessor:

class _LazyPlatformRouterWithStore(LazyPlatformRouter):
    def __init__(self, *args, proposal_store: ProposalStore | None = None, **kwargs):
        super().__init__(*args, **kwargs)
        self._proposal_store = proposal_store

    def proposal_store_for_tenant(self, tenant_id: str) -> ProposalStore | None:
        return self._proposal_store

Works because the framework's dispatch is duck-typed (hasattr), but every adopter wiring LazyPlatformRouter will hit the same wall and write the same subclass. Strict parity with PlatformRouter would resolve.

Proposed fix

Mirror PlatformRouter's shape on LazyPlatformRouter:

• Add proposal_stores: Mapping[str, ProposalStore] | None = None to __init__
• Persist as self._proposal_stores: dict[str, ProposalStore]
• Add proposal_store_for_tenant(self, tenant_id: str) -> ProposalStore | None: return self._proposal_stores.get(tenant_id)
• Match the eager router's cross-store consistency check (finalize_supported and tenant_id not in self._proposal_stores → ValueError)

Single-store-across-tenants adopters can still pass {tid: shared_store for tid in tenants} or — better — accept a proposal_store_factory: Callable[[str], ProposalStore] (the same shape as factory= for platforms). The factory shape composes more naturally with the lazy router's per-tenant-on-first-request philosophy.

Context

Hit while wiring a Postgres-backed ProposalStore in prebid/salesagent (PR #390) to unblock the media_buy_seller/proposal_finalize/create_media_buy storyboard step. Filed concurrent issue on the lifecycle gap that storyboard exposes (separate from this).