fix(lint): extract repeated strings as constants to satisfy goconst

[uwe-mayer]

Summary

Fixes goconst lint errors by extracting repeated string literals as constants:

• controller/careinstruction/metrics.go: Extract metric label keys (care_instruction, namespace, garden_namespace, shoot_name) as package-level constants
• controller/shoot/auth.go: Extract "config.yaml" ConfigMap data key as a constant

These were flagged by the updated golangci-lint goconst rule.

[Copilot]

Pull request overview

This PR addresses new golangci-lint goconst findings by replacing repeated string literals with package-level constants, keeping behavior unchanged while reducing lint noise.

Changes:

• Extracted Prometheus metric label keys into constants in the CareInstruction metrics package.
• Extracted the ConfigMap.Data key "config.yaml" into a constant in Shoot OIDC/auth configuration code.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
controller/shoot/auth.go	Replaces repeated "config.yaml" ConfigMap data key usages with a single package constant.
controller/careinstruction/metrics.go	Replaces repeated metric label key strings with package-level constants for Prometheus label definitions/usage.

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[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/cloudoperators/shoot-grafter/controller/careinstruction	0.00% (ø)
github.com/cloudoperators/shoot-grafter/controller/shoot	0.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/cloudoperators/shoot-grafter/controller/careinstruction/metrics.go	0.00% (ø)	0	0	0
github.com/cloudoperators/shoot-grafter/controller/shoot/auth.go	0.00% (ø)	0	0	0

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.