hosting: a resolver returning TenantResolution(None, source) leaves tenant_source set with no tenant bound

[antosubash]

Problem

request.state.tenant_source is documented as None when no tenant was bound, and the built-in paths honour that. The (tenant_id, source) tuple path in _normalise and the tenants module's resolver both blank the source when tenant_id is None. A custom resolver that returns a TenantResolution directly skips that: _normalise passes the object through unchanged.

if isinstance(result, TenantResolution):
    return result          # TenantResolution(None, "header") stays source="header"

So a resolver returning TenantResolution(None, "header", ("X-Tenant-ID",)) leaves the request with tenant_id=None, tenant_source="header". Code that branches on tenant_source (for example to choose between Vary: <header> and Cache-Control: private, the use case in #367) then believes a tenant was resolved from the header when none was.

Reproduction

Found by independent QA on #384 and still on main at 7db61a0 (framework/hosting/simple_module_hosting/_tenant.py, _normalise): register app.state.tenant_resolver returning TenantResolution(None, "header", ("X-Tenant-ID",)) and read request.state in a route. It shows tenant_id=None, tenant_source='header'.

Expected

_normalise applies the same rule to every result shape: when tenant_id is None, the source is None. The vary names are kept, since the header's absence was still an input. For example:

if isinstance(result, TenantResolution):
    return result if result.tenant_id is not None else TenantResolution(None, None, result.vary)

Tests to add

• A resolver returning TenantResolution(None, "header", ("X-Tenant-ID",)) → tenant_source is None, and the response still has Vary: X-Tenant-ID.