fix: restore tenant context for error channel in multi-tenant setup

[dgafka]

Why is this change proposed?

Fixes #676. In a multi-tenant setup, when a polling consumer fails and the message is routed to a DBAL Dead Letter error channel, DbalDeadLetterHandler crashes with Lack of context about tenant in Message Headers. By the time the error-channel interceptor catches the exception, the interceptor chain has already unwound — the tenant was deactivated and the propagated-headers context popped — so HeaderBasedMultiTenantConnectionFactory::createContext() cannot resolve any connection. Failed messages can never reach the dead letter, and the consumer crashes instead of self-healing.

Description of Changes

• ErrorChannelService now re-establishes the failed message's header context (via MessageHeadersPropagatorInterceptor::storeHeaders) around the send to the error channel, so the tenant header of the failed message is visible again to any tenant-aware component downstream
• The fix is placed in ErrorChannelService (not in the DBAL dead letter handler) so custom error handlers and other integrations regain tenant context as well
• Added integration test proving a failed message lands in the dead letter table of the tenant it belongs to, and not in other tenants' databases

No configuration changes are needed — existing multi-tenant setups start working:

final class Configuration
{
    #[ServiceContext]
    public function multiTenant(): MultiTenantConfiguration
    {
        return MultiTenantConfiguration::create(
            tenantHeaderName: 'tenant',
            tenantToConnectionMapping: [
                'tenant_a' => 'tenant_a_connection',
                'tenant_b' => 'tenant_b_connection',
            ],
        );
    }

    #[ServiceContext]
    public function errorHandling(): ServiceConfiguration
    {
        return ServiceConfiguration::createWithDefaults()
            ->withDefaultErrorChannel('dbal_dead_letter');
    }
}

When a handler for tenant_a fails, the message is now stored in ecotone_error_messages in tenant A's database.

sequenceDiagram
    participant Consumer as Polling Consumer
    participant Ack as AcknowledgeInterceptor
    participant Error as ErrorChannelInterceptor
    participant Headers as HeadersPropagator
    participant Handler
    participant DLQ as DbalDeadLetterHandler

    Consumer->>Ack: message (tenant: tenant_a)
    Ack->>Error: proceed
    Error->>Headers: proceed (push tenant header context)
    Headers->>Handler: handle
    Handler--xHeaders: exception (context popped on unwind)
    Headers--xError: exception
    Note over Error: NEW: restore failed message's<br/>header context around send
    Error->>DLQ: store failed message
    DLQ->>DLQ: resolve tenant_a connection ✔
    Error-->>Ack: handled
    Ack->>Ack: accept()

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