Detect struct comparisons that bypass +noKrtEquals markers

[Copilot]

The linter missed cases where struct-typed fields are compared using == instead of delegating to .Equals(), allowing Go's default comparison to ignore +noKrtEquals markers on nested fields.

Problem

type listenerPolicy struct {
    proxyProtocol *int
    // +noKrtEquals reason: internal state
    http *HttpListenerPolicyIr
}

type ListenerPolicyIR struct {
    defaultPolicy listenerPolicy
}

// BUG: Compares ALL fields of listenerPolicy, including http
func (l ListenerPolicyIR) Equals(other ListenerPolicyIR) bool {
    return l.defaultPolicy == other.defaultPolicy  // Now flagged
}

When == compares structs, it compares every field regardless of markers. The linter previously only checked that fields are used, not how they're compared.

Changes

• Added checkStructComparisons() to detect ==/!= on struct-typed fields
• Traverses type info to identify struct types and check for marked fields
• Reports diagnostic when comparison would bypass +noKrtEquals/+krtEqualsTodo markers
• Suggests .Equals() delegation as the correct pattern

Test Coverage

• Struct fields with +noKrtEquals markers compared via == (flagged)
• Struct fields with +krtEqualsTodo markers compared via == (flagged)
• Proper delegation using .Equals() (passes)
• Both exported and unexported field scenarios

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Investigate kgateway-dev/kgateway#13271 (comment)</issue_title>
<issue_description>Determine why the linter didn't catch a nested field within the listener policy IR struct:

type ListenerPolicyIR struct {
	ct            time.Time
	defaultPolicy listenerPolicy
	perPortPolicy map[uint32]listenerPolicy

	NoOrigin bool // +noKrtEquals reason: When set to true, suppress source reporting metadata from
	// ListenerPolicy specific fields that are irrelevant to the (now deprecated) HTTPListenerPolicy. Remove when HTTPListenerPolicy is removed.
}

type listenerPolicy struct {
	proxyProtocol                 *anypb.Any
	perConnectionBufferLimitBytes *uint32
	http                          *HttpListenerPolicyIr
}
```</issue_description>

<agent_instructions>Determine whether the code I inlined in the issue is a gap the linter doesn't handle correctly</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Investigate https://github.com/kgateway-dev/kgateway/pull/13271#issuecomment-3739843830 #3

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