`abort "msg"` reason only preserved with explicit `reroute_dropped: true`

Vector version: 0.58.0

Severity: Major (diagnostic info loss)

Description:

When a VRL program calls abort "diagnostic message", the message text is preserved in metadata.dropped.message only if reroute_dropped: true is set AND a sink subscribes to .dropped.

Source code (confirmed):
src/transforms/remap.rs:395:

if self.reroute_dropped {
    vec![
        default_output,
        TransformOutput::new(...).with_port(DROPPED),
    ]
} else {
    vec![default_output]
}

If reroute_dropped is false, no output port for dropped events is created. The original event is silently discarded. Operators see only "Event mapping aborted." reason in internal logs but lose the abort message text.

Reproduction:

transforms:
  parse:
    type: remap
    inputs: [source]
    drop_on_abort: true
    # reroute_dropped NOT set
    source: |
      if !is_ipv4(string!(.col_ip)) {
        abort "col_ip invalid: " + string!(.col_ip)
      }

sinks:
  out:
    type: console
    inputs: [parse]

When col_ip = "garbage":

• Event is dropped
• Internal logs show only "Event mapping aborted."
• The string "col_ip invalid: garbage" is lost (not in logs, not in sinks)

Vector runtime tests confirm:

• Sink file: not created (no events reach it)
• Log output: reason="Event mapping aborted." (no abort text)

Expected:

Abort messages should be preserved somewhere even without reroute_dropped. Options:

• Emit WARN log on abort when reroute_dropped=false, including the abort message text
• Document explicitly: "abort messages are lost without reroute_dropped: true"

Workaround:

Always set reroute_dropped: true and add a sink subscribed to .dropped:

transforms:
  parse:
    type: remap
    inputs: [source]
    drop_on_abort: true
    reroute_dropped: true  # explicit
    source: |
      if !is_ipv4(string!(.col_ip)) {
        abort "col_ip invalid: " + string!(.col_ip)
      }

sinks:
  out:
    type: console
    inputs: [parse]
  dl:
    type: file
    inputs: [parse.dropped]
    path: /tmp/dl.jsonl

Impact:

Production operators debugging aborted events have no diagnostic text beyond "abort" reason. Root cause investigation requires adding reroute_dropped: true to every transform in the pipeline, which is not enforced.

Confirmed in source: src/transforms/remap.rs:395 (Vector 0.58.0)

Start at src/transforms/remap.rs:395 and trace the output handling for aborted events when reroute_dropped is false. Reproduce the behavior with the YAML configuration in the issue and compare the internal log output with the abort message. Review the existing Vector runtime tests mentioned in the report. Done should establish either preserved abort text without rerouting or explicit documentation of its loss.