feat(sources): add disconnect_mode option to TCP sources

[tronboto]

Summary

Adds a disconnect_mode configuration option to TCP sources (socket, logstash, fluent, syslog, statsd) that controls how connections are closed on shutdown or when max_connecti on_duration_secs elapses.

The two modes are:

• drain (the default and existing behaviour) - sends a FIN and waits for the client to close.
• abort - sets SO_LINGER=0 to force close immediately with RST.

The original motivation is "write-only" clients such as Serilog.Sinks.Network that never read from the socket and ther
efore cannot detect a graceful shutdown. Additionally, the "reference implementations" for the beats (logstash) and fluent protocols also use an abort style shutdown:

• Logstash beats input
• Fluentd server

All sources default to drain, so this is opt-in with no behaviour change for existing configs.

Vector configuration

sources:
  tcp_in:
    type: socket
    mode: tcp
    address: "0.0.0.0:9000"
    shutdown_timeout_secs: 5
    disconnect_mode: abort  # omit for default drain behaviour
sinks:
  out:
    type: console
    inputs: [tcp_in]
    encoding:
      codec: text

How did you test this PR?

• Manual testing with netcat and a write-only Python client against both modes, verifying FIN and RST behaviour using tcpdump.
• A new unit test added to sources::socket:
  • tcp_disconnect_mode_abort_on_shutdown - verifies RST is received immediately on shutdown.

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

Notes

• Please read our Vector contributor resources.
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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Apply abort mode on the force-timeout path

With disconnect_mode = abort, if shutdown_timeout_secs has already elapsed (for example it is set to 0, or the connection is stuck processing a batch until the force timeout), the tripwire branch exits without calling close_socket, so SO_LINGER=0 is never applied and clients can see the old FIN behavior instead of the configured RST. Route this branch through close_socket(..., disconnect_mode) before breaking.

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[chatgpt-codex-connector]

Update generated docs for disconnect_mode

This adds a user-facing component configuration field, but the generated component reference files were not updated (rg disconnect_mode website/cue/reference/components/sources/generated returns no entries). The new option would be absent from the published reference and generated-docs checks should fail; please run make generate-docs and commit the generated CUE updates.

AGENTS.md reference: AGENTS.md:L114-L118

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