Scope the Postgres enum registrations to the EF Core service provider

[dmytro-khmara]

Summary

Makes RawSqlEnumParameterTests stop failing intermittently in CI with 42804: column "Sport" is of type sport but expression is of type text.

The store-type lookup lived in a process-wide static StringEnumPgTypeRegistry, while EF Core memoises the first FindMapping(typeof(TEnum), storeType: null) result in a cache owned by the internal service provider. Because StrEnumNpgsqlOptionsExtension keyed every configuration to the same provider, all four integration-test contexts shared one type-mapping cache. Whichever context built its model first won: if one that had not registered an enum got there first, EF memoised the miss and served it to the context that had registered, so raw-SQL parameters fell back to text. Test classes run in parallel, hence the flakiness.

The fix scopes the registrations to the service provider that owns the cache:

• StringEnumPostgresEnumRegistrar accumulates its registrations instead of writing to a static, and the options extension carries them.
• NpgsqlStringEnumTypeMappingSourcePlugin takes that map by constructor and resolves against it, so a plugin instance only ever sees its own context's registrations.
• StringEnumPgTypeRegistry is deleted; StringEnumPgTypeKey holds the shared key format.

One behaviour change worth noting: HasPostgresStringEnum / MapStringEnumAsPostgresEnum no longer feed the raw-SQL parameter path, because model-level configuration runs inside OnModelCreating — after EF has already memoised the (TEnum, null) lookup. That path never worked reliably; it is the race this PR removes. Up-front registration via UseStringEnumsAsPostgresEnums(r => r.MapStringEnum<TEnum>()) remains the supported way to bind string enums on the raw-SQL path, exactly as RawSqlEnumParameterTests already does. Property-driven mapping is unaffected.

Test plan

• Integration suite run 15 times consecutively: 15 passed, 0 failed. Before the fix the same suite failed 2 of 5 runs, and 1 of 10 after only keying the provider hash code
• dotnet test — 29 unit and 6 integration tests pass
• docker build --target test . passes
• New StrEnumNpgsqlOptionsExtensionTests cover provider keying: same registrations share a provider; differing registrations, differing enum names, and differing enums do not
• NpgsqlStringEnumTypeMappingSourcePluginTests updated to construct the plugin with explicit registrations rather than mutating global state