[Heartbeat] Test that omitted default fields fall back to defaults

[shahzad31]

Summary

Adds unit tests for the http, tcp and icmp active monitors proving that when a configuration field is either fully absent or explicitly null, Heartbeat still applies the exact same default value — and that explicit non-default values still override.

Fields covered:

• http: check.request.method, max_redirects, response.include_body, response.include_headers, ipv4, ipv6, mode, timeout
• tcp: timeout, ipv4, ipv6, mode, proxy_use_local_resolver
• icmp: timeout, wait, ipv4, ipv6, mode

Why

This is the safety proof for a change in Kibana + the Synthetics integration that stops emitting monitor fields whose value already equals the Heartbeat default (to shrink the agent payload):

• [Synthetics] Prevent setting default values in yaml output kibana#241818
• [Synthetics] Guard default monitor fields in agent stream templates integrations#20342

These tests guarantee that dropping such fields from the agent stream is behavior-neutral for the running monitor.

Test plan

• go test ./heartbeat/monitors/active/{http,tcp,icmp}/ -run TestOmittedDefaultsFallBackToHeartbeatDefaults passes

Made with Cursor

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @shahzad31? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
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