Make InMemorySessionStore.append idempotent by uuid

[shashvat-singham]

Problem

SessionStore.append documents uuid as an idempotency key:

Most entries carry a stable uuid that adapters should treat as an idempotency key (upsert / ignore-duplicate). Entries without a uuid (e.g. titles, tags, mode markers) should be appended without dedup.

InMemorySessionStore.append extends unconditionally, so a re-appended entry is stored twice:

await store.append(KEY, [{"type": "user", "uuid": "a"}, {"type": "user", "uuid": "b"}])
await store.append(KEY, [{"type": "user", "uuid": "b"}, {"type": "user", "uuid": "c"}])
await store.load(KEY)
# uuids: ['a', 'b', 'b', 'c']   -- 'b' duplicated

This is reachable rather than theoretical. TranscriptMirrorBatcher retries failed batches (3 attempts), and its own docstring says a retried batch "may partially overlap a prior partial write" — which is precisely the case the idempotency clause exists for. The overlapping entries then show up twice in the resumed transcript.

Change

Skip entries whose uuid is already stored; append entries without a uuid verbatim, as the contract requires.

uuids: ['a', 'b', 'c']              # after the two overlapping appends
len(...) == 2                       # two no-uuid entries, still both kept

On the conformance suite (deliberately not changed)

I first added an idempotency check to run_session_store_conformance, then backed it out. It fails the MinimalStore fixtures in tests/test_session_store_conformance.py, which use a plain .extend() — so the contract's "should" reads as advisory guidance for adapter authors rather than a mandatory conformance requirement, and enforcing it would be a breaking change for existing adapters.

That does leave the clause untested, so an adapter can pass conformance either way. If you'd like it enforced, I'm happy to send that as a separate PR (it would need the in-tree MinimalStore fixtures updated too) — but it seemed like your call, not mine.

Tests

test_append_is_idempotent_by_uuid and test_append_does_not_dedup_entries_without_uuid. The first fails on main.

$ pytest tests/test_session_store_conformance.py -q -k asyncio
10 passed

(The [trio] parametrisations fail identically before and after on my machine — no trio backend installed — so I've filtered to asyncio here.)