Add send_keepalive_packet option to out_forward plugin

[2ZZ]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:

This PR adds the send_keepalive_packet configuration parameter to the out_forward plugin to enable TCP keepalive (SO_KEEPALIVE) on forward output connections.

Background:
The existing keepalive parameter only enables connection pooling/reuse, not TCP-level keepalive probes. This can lead to accumulation of dead connections in CLOSE-WAIT state.

Changes:

• Added send_keepalive_packet config parameter (default: false)
• Pass the parameter to both socket_create_tcp and socket_create_tls calls
• Added tests to verify the parameter works correctly

Related:
This follows the same pattern as #2352 which added the same feature to in_forward.

Docs Changes:

Release Note:

• out_forward: Add send_keepalive_packet option

[Watson1978]

@2ZZ Can you add DCO sign in your commit and fix tests ?

[2ZZ]

@2ZZ Can you add DCO sign in your commit and fix tests ?

Fixed and signed.

One test failed but it doesn't look related to my change (https://github.com/fluent/fluentd/actions/runs/22176445014/job/64126271546?pr=5262#step:6:5177)

[Watson1978]

I think send_keepalive_packet should only be set to true when keepalive is set to true.

Like:

fluentd/lib/fluent/plugin/in_forward.rb

Lines 155 to 157 in 1327bed

	if @send_keepalive_packet && @deny_keepalive
	raise Fluent::ConfigError, "both 'send_keepalive_packet' and 'deny_keepalive' cannot be set to true"
	end

Can you fix this and add tests for that, like following ?

fluentd/test/plugin/test_in_forward.rb

Lines 107 to 114 in 1327bed

	test 'both send_keepalive_packet and deny_keepalive cannot be enabled' do
	assert_raise(Fluent::ConfigError.new("both 'send_keepalive_packet' and 'deny_keepalive' cannot be set to true")) do
	create_driver(config_auth + %[
	send_keepalive_packet true
	deny_keepalive true
	])
	end
	end

[2ZZ]

I think send_keepalive_packet should only be set to true when keepalive is set to true.

Like:

fluentd/lib/fluent/plugin/in_forward.rb

Lines 155 to 157 in 1327bed

	if @send_keepalive_packet && @deny_keepalive
	raise Fluent::ConfigError, "both 'send_keepalive_packet' and 'deny_keepalive' cannot be set to true"
	end

Can you fix this and add tests for that, like following ?

fluentd/test/plugin/test_in_forward.rb

Lines 107 to 114 in 1327bed

	test 'both send_keepalive_packet and deny_keepalive cannot be enabled' do
	assert_raise(Fluent::ConfigError.new("both 'send_keepalive_packet' and 'deny_keepalive' cannot be set to true")) do
	create_driver(config_auth + %[
	send_keepalive_packet true
	deny_keepalive true
	])
	end
	end

Makes sense, updated now & tests passed