feat(checkpoints): Add StoreDimensionProvider with store.country (#3969)

### Description

Adds the store as a third dimension source, with one dimension for now:
`store.country`.

Note that what we get in Android is a different code than in iOS. For
example, we get the 2-letter `US` instead of the 3-letter `USA` as in
iOS. In this PR, I try to mimick the format in iOS, so we can
consolidate the rules.

It fetches through `awaitStorefrontCountryCode` rather than reading the
cached property, so the dimension is available on the first checkpoint
of a session instead of missing until the billing client happens to
connect. A store that can't answer contributes nothing instead of
failing the snapshot — that is a routine outcome (the Galaxy Store never
answers), while a failure would abort the evaluation and take an
otherwise resolvable checkpoint down with it.

Similar to RevenueCat/purchases-ios#7399.

Also fixes `RulesDimensionResolver` nesting an empty object for a source
with nothing to contribute. An empty object is truthy in JSON Logic, so
`{"var": "store"}` would have read as present for a customer whose store
never answered — reachable for the first time now that a provider
routinely returns nothing.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes checkpoint rule evaluation semantics (new dimension and
absent-vs-empty namespace behavior) and adds an async storefront fetch
on the evaluation path, though store failures are isolated and do not
abort snapshots.
> 
> **Overview**
> Checkpoint **local rules** can now read **`store.country`** (ISO
3166-1 alpha-3, e.g. `USA`) so predicates can align with iOS. Android
alpha-2 storefront codes are converted at evaluation time; unmapped or
invalid codes are omitted.
> 
> **`StoreDimensionProvider`** is registered on
**`LocalRulesEvaluator`** and resolves the country via
**`awaitStorefrontCountryCode()`** on each evaluation (not a cached
snapshot), so the first checkpoint in a session can still get a value
once the store responds. Store errors, missing answers, or teardown
mid-evaluation yield **no store namespace** instead of failing the whole
dimension snapshot.
> 
> **`RulesDimensionResolver`** no longer nests an empty object when a
source has nothing to contribute (including empty **custom** variables).
Empty objects are truthy in JSON Logic, so absent namespaces correctly
mean “store/custom not present” for predicates like `{"var": "store"}`.
> 
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<!-- /CURSOR_SUMMARY -->

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>