OIDC discovery fails for issuers with a trailing slash

## OIDC discovery fails for issuers with a trailing slash

`OidcDiscovery::fetchMetadata()` strips the trailing slash from the issuer before discovery:

```php
$issuer = rtrim($issuer, '/');
```

The normalized issuer is then used to validate the `issuer` returned in the discovery metadata.

This causes discovery to fail for providers that use a trailing slash in their canonical issuer, such as Authentik:

```text
Expected: https://auth.example.com/application/o/mcp
Got:      https://auth.example.com/application/o/mcp/
```

The trailing slash should only be removed when building the discovery URL. The original issuer should be preserved for validation.

[Related PR](https://github.com/modelcontextprotocol/php-sdk/pull/506)

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OIDC discovery fails for issuers with a trailing slash #507