refactor(fqdn): encapsulate FQDN template logic into TemplateEngine

[ivankatliarchuk]

What does it do ?

• Introduced fqdn.TemplateEngine (value type) encapsulating templates and the combineFQDN flag
• Replaced the free function CombineWithTemplatedEndpoints with TemplateEngine.CombineWithEndpoints method
• Made routeGroup implement kubeObject (embed metav1.TypeMeta/ObjectMeta), eliminating the deprecated ExecuteFQDN path
• Extracted template helper functions into fqdn/functions.go with their own tests

Follow-up

• support templating for all sources
• support multiple --fqdn-template=, instead of current single --fqdn-template=""
• all sources - 90+% coverage
• add functions (to complement functionality)
  • resolve host to ip feat(gateway): add --resolve-gateway-load-balancer-hostname flag #6286
  • convert to ptr feat!: generalize PTR record support from rfc2136 to all providers #6232

Motivation

One place to change. Adding a fourth template field, changing combine semantics, adding a new template function - all changes land in fqdn/ and propagate automatically to all 10+ sources.

FQDN template handling was duplicated inline across ~10 source files — each called a free function with inconsistent guarding and accessed the engine's internals directly. There was no single place to reason about template configuration or behavior.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

[coveralls]

Pull Request Test Coverage Report for Build 23435131007

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 332 unchanged lines in 15 files lost coverage.
• Overall coverage increased (+0.2%) to 78.224%

---

Files with Coverage Reduction	New Missed Lines	%
fake.go	1	96.08%
integration/toolkit/toolkit.go	2	0.0%
pod.go	5	97.18%
node.go	12	90.48%
istio_gateway.go	16	89.88%
execute.go	19	76.3%
openshift_route.go	19	83.74%
contour_httpproxy.go	20	83.72%
ingress.go	21	88.42%
store.go	28	63.87%

Totals
Change from base Build 23401388822:	0.2%
Covered Lines:	16409
Relevant Lines:	20977

---

💛 - Coveralls

[mloiseleur]

Using templates can make you think it's an array of fqdn.Template.
Since it's a fqdn.TemplateEngine, shouldn't the variable be named templateEngine ?

[ivankatliarchuk]

Thanks for flagging. Changed everywhere.

[ivankatliarchuk]

#6292 (comment)

[ivankatliarchuk]

I renamed fqdn package to templateenginge

[mloiseleur]

Suggested change

	package templateegine
	package template

There is a typo in the directory name & package name.
It should be at least templateengine.
Since the type is now named Engine, the package could be named template so it would be called in all sources with template.Engine.
Wdyt ?

[ivankatliarchuk]

Yeah. Make sense

[ivankatliarchuk]

Changed

[ivankatliarchuk]

moved template engine test utils under source/template/testutil/testutil.go. Source tests no need to be aware of template engine creation cbf39cf

[mloiseleur]

Looks very clear now 👍.
/lgtm

[ivankatliarchuk]

/approve

[k8s-ci-robot]

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