fix(mqtt source): pass client certificates to rumqttc for mTLS

[mr-]

Summary

This PR fixes a bug in the MQTT source where user-provided TLS client certificates (crt_file / key_file) were being silently ignored, breaking mTLS connections to strict brokers like AWS IoT Core.

Vector configuration

sources:
  aws_iot_subscriber:
    type: "mqtt"
    host: "foo-ats.iot.eu-central-1.amazonaws.com"
    port: 8883
    client_id: "MARTIN"

    topic:
      - "v1/sensors"


    tls:
      enabled: true
      ca_file:  "AmazonBundle.pem"
      crt_file: "AWS-IoT-Client.crt"
      key_file: "AWS-IoT-Client-pkcs8.key"
    decoding:
      codec: "bytes"


sinks:
  console_output:
    type: "console"
    inputs:
      - "aws_iot_subscriber"
    target: "stdout"
    encoding:
      codec: "text"

How did you test this PR?

It was tested manually by providing valid certificates.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

Not sure. Not unless people depended on certificates being ignored.

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[mr-]

@vectordotdev/vector when debugging this, I've noticed that all errors are silently ignored.
I've added logs now, but I'm not sure if there's more error handling that should be done. As far as I know, rumqttc's EventLoop/poll will try to reconnect, but not all errors are transient.

[mr-]

I have read the CLA Document and I hereby sign the CLA

[mr-]

Hey @vectordotdev/vector, can I have a review please? It's only a 1 line change now :)

[thomasqueirozb]

Thank you!