fix(topology): prevent source pump deadlock during sink config reload

[joshcoughlan]

Summary

During config reload, when a sink in wait_for_sinks is being changed, remove_inputs previously sent Pause to the upstream fanout. This caused the source pump to block in wait_for_replacements — waiting for a Replace message that only arrives after connect_diff, which runs after shutdown_diff completes. Since shutdown_diff waits for the old sink to finish, this created a circular dependency that stalled all sources.

The fix uses Remove instead of Pause for sinks in wait_for_sinks during reload, and correspondingly uses Add instead of Replace when reconnecting them in connect_diff. This allows the source pump to continue sending events to other sinks while the old sink drains, breaking the circular dependency.

This is buffer-type agnostic — it fixes the deadlock for memory and disk buffered sinks alike, and only affects the reload path.

Vector configuration

sources:
  source_socket:
    type: socket
    address: 0.0.0.0:9000
    mode: tcp
    decoding:
      codec: json

sinks:
  sink_http:
    type: http
    inputs:
      - source_socket
    uri: http://localhost:9222
    method: get
    encoding:
      codec: json

How did you test this PR?

• Added two new regression tests:
  • topology_reload_conflicting_sink_does_not_stall — verifies reload completes when a sink has conflicting resources
  • topology_reload_reuse_buffer_does_not_stall — verifies reload completes when buffer is reused (SIGHUP-style)
• Both tests use a 10-second timeout to detect the deadlock
• All 11 reload tests pass (9 existing + 2 new)
• make check-clippy passes with all features (Docker x86_64 environment)
• make test passes (Docker x86_64 environment)
• make check-fmt passes

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?

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

References

• Closes: Vector fails to reload sink with failed events vectordotdev/vector#24125
• Related: fix(topology): Fix for issue causing stalling on shutdown for sinks configured w/ disk buffers vectordotdev/vector#24949
• Related: Reload buffer configuration vectordotdev/vector#18933