feat(server): lazy idempotency backend factory (JS #2136 parity)

[bokelley]

Gap

JS SDK adcp-client#2136 added createLazyBackend() for deferred (async-resolved) idempotency backend construction — for adopters whose Redis/Pg client resolves asynchronously from app infra.

Python idempotency exposes only MemoryBackend and PgBackend (src/adcp/server/idempotency/__init__.py); the "lazy" matches in backends.py refer to lazy expiry, not deferred construction. No factory wrapper exists.

Scope

Add a lazy backend wrapper around IdempotencyBackend with async first-use resolution and opt-in clear_all, mirroring JS createLazyBackend.

Notes

• Reference: adcontextprotocol/adcp-client#2136.
• Narrow use case (async-resolved Pg/Redis pool); lowest priority of the parity gaps.
• Single-agent triage verification.
• Filed no-triage.