docs(contributor): GetDomainFilter contract and ZoneCache blueprint for provider contributors

[ivankatliarchuk]

What does it do ?

• Adds a GetDomainFilter section covering the controller integration, the static anti-pattern with a concrete example, the correct dynamic pattern with worked scenarios, zone name formatting pitfalls, and test requirements. Adds a Provider Blueprints section covering ZoneCache usage pattern and the single --zones-cache-duration flag.

• Clarify ZoneCache godoc to cover non-obvious expiry and disabling behaviour

Motivation

• Most provider implementations repeatedly made the same two mistakes: echoing config.DomainFilter back from GetDomainFilter() instead of querying live zones, and reimplementing zone caching from scratch instead of using the shared blueprint. Neither pattern was documented anywhere.

More

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

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[coveralls]

Pull Request Test Coverage Report for Build 22779821196

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.165%

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Totals
Change from base Build 22540459008:	0.0%
Covered Lines:	16057
Relevant Lines:	20283

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💛 - Coveralls

[clwluvw]

@ivankatliarchuk - How should we handle the situation where we have authoritative zones for the provider, such as a.com and b.com, but the user provides a domain filter for c.com? If GetDomainFilter() returns an empty list, should we assume that c.com can be reconciled?

[ivankatliarchuk]

@ivankatliarchuk - How should we handle the situation where we have authoritative zones for the provider, such as a.com and b.com, but the user provides a domain filter for c.com? If GetDomainFilter() returns an empty list, should we assume that c.com can be reconciled?

I answered it here #6234 (comment)

[ivankatliarchuk]

Added to docs an example with, a,b and c domains.