#4668 — tenant-aware RebuildSingleStreamAsync overloads on AdvancedOperations

[jeremydmiller]

Closes #4668.

Adds two new overloads on AdvancedOperations:

Task RebuildSingleStreamAsync<T>(string streamKey, string tenantId, CancellationToken token = default)
Task RebuildSingleStreamAsync<T>(Guid   streamId,  string tenantId, CancellationToken token = default)

Backwards-compatibility shape

New methods, not default-argument additions on the existing signatures.
Per the issue's explicit request — keeping (string id, CT) and (Guid id, CT)
bind-compatible at the call site for every existing caller. No optional
argument anywhere on this path.

Implementation

Overload	Session	Notes
(string streamKey, string tenantId, CT)	LightweightSession(tenantId)	Mirrors the existing (string, CT) overload's session shape, scoped to the tenant.
(Guid streamId, string tenantId, CT)	LightweightSession(new SessionOptions { ConcurrencyChecks = Disabled, TenantId = tenantId })	Mirrors the existing Guid overload's ConcurrencyChecks.Disabled posture, with TenantId set on the options.

Tenant id flows through both the AggregateStreamAsync read and the
projected-document upsert.

Tests

src/MultiTenancyTests/rebuild_single_stream_tenant_overloads.cs — four
tests covering the matrix of tenancy style × stream-identity:

Test	Tenancy	Stream identity
conjoined_rebuild_by_guid_with_tenant_id	Conjoined	Guid
conjoined_rebuild_by_string_key_with_tenant_id	Conjoined	String
per_database_rebuild_by_guid_with_tenant_id	MultiTenantedWithSingleServer	Guid
per_database_rebuild_by_string_key_with_tenant_id	MultiTenantedWithSingleServer	String

Each test:

1. Seeds two tenants with different event shapes (e.g. 3× AEvent vs 2× BEvent).
2. Calls the new overload to rebuild tenant A's stream.
3. Asserts tenant A's projection reflects only A's events.
4. Repeats for tenant B.
5. (Conjoined-Guid only) pins cross-tenant isolation: loading tenant B's
   stream id through a tenant A query session returns null.

The per-database variants exercise MultiTenantedWithSingleServer
routing end-to-end (a host + AddMarten + ApplyAllDatabaseChangesOnStartup
bootstrap), so the rebuild's session has to route to each tenant's
physical database for both the event load and the upsert.

Verification

Local on net9.0:

Step	Result
Build (Marten + MultiTenancyTests)	clean
4 new tests	all pass, total 1.7s

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