feat: trigger shoot reconciliation after OIDC updates

[Zaggy21]

Summary

Trigger Shoot reconciliation automatically when OIDC AuthenticationConfiguration ConfigMap content changes.

Behavior

• When ConfigMap content changes + Shoot spec unchanged: Annotates Shoot → immediate reconciliation
• When ConfigMap content changes + Shoot spec needs update: Updates spec only → spec change triggers reconciliation
• When nothing changes: No action taken

Docs
Added new section "OIDC Configuration Updates and Shoot Reconciliation" to README.md.

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/cloudoperators/shoot-grafter/api/v1alpha1	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/api/v1alpha1/zz_generated.deepcopy.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.

Changed unit test files

• github.com/cloudoperators/shoot-grafter/controller/shoot/auth_test.go
• github.com/cloudoperators/shoot-grafter/controller/shoot/shoot_controller_test.go