Warn when a JwtBearer scheme is configured with multiple issuers sharing one signing-key pool

`JwtBearerConfigureOption`s uses each `SigningKeys[].Issuer` label only to select which key to build, then flattens all of them into one `TokenValidationParameters.IssuerSigningKeys` (JwtBearerConfigureOptions.cs:80-92). Because IdentityModel validates `iss` and the signature independently, any configured key can sign for any configured issuer. That's the documented flat-trust-pool contract for a single scheme, but the per-key Issuer label reads like a runtime binding it isn't.

When the binder produces keys for more than one issuer, log a warning. Something like _Scheme {Scheme} has {Count} configured issuers sharing one signing-key pool. Any configured key can sign for any configured issuer. Use separate schemes if these issuers are distinct trust boundaries._