Limit AWS DNS provider validation to primary provider

[wpross]

How to categorize this PR?

/area quality
/kind enhancement
/platform aws

What this PR does / why we need it:
This PR limits the validation of DNS provider secrets referenced in the shoot spec to the primary provider. All other provider secrets are validated in the https://github.com/gardener/gardener-extension-shoot-dns-service that they are used in.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

[github-actions]

This change refines the DNS provider validation logic in Gardener's shoot admission validator to only validate credentials for primary DNS providers, improving performance and reducing unnecessary validation overhead.

Walkthrough

• Bug Fix: Modified DNS provider validation to skip non-primary providers by checking the Primary field before performing secret validation
• Test: Added comprehensive test coverage for primary provider filtering scenarios, including edge cases with multiple providers, mixed provider types, and nil Primary values
• Test: Updated existing test cases to explicitly set Primary: true on DNS providers to maintain current validation behavior

_{Model: claude-sonnet-4-20250514 | Prompt Tokens: 2965 | Completion Tokens: 124}

[MartinWeindel]

lgtm

[AndreasBurger]

lgtm

[gardener-prow]

LGTM label has been added.

Details

Git tree hash: 7ee7e8b8f8af77785a565913eb0d005c429ed39e

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AndreasBurger

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• OWNERS [AndreasBurger]

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