fix: improve github_organization_custom_properties resource

[mkushakov]

Summary

Fix several bugs and improve the github_organization_custom_properties resource to handle edge cases correctly and align with GitHub API behavior.

Resolves #2806

Changes

Bug Fixes

• Remove broken CustomizeDiff: The existing CustomizeDiff referenced a slug field that does not exist in the schema, causing unnecessary plan diffs. Removed entirely.
• Fix Update double-read: The Update function was reading the resource state twice unnecessarily. Removed the redundant read.
• Fix missing 404 handling in Read: If a custom property is deleted outside of Terraform, the Read function now gracefully removes it from state instead of returning an error.
• Use GetOk for optional fields: Changed allowed_values and default_value reads to use GetOk instead of Get to correctly distinguish between unset and empty values.

Improvements

• Add ForceNew to immutable fields: property_name and value_type cannot be changed on an existing custom property — they now force recreation.
• Make value_type Required: This field is required by the GitHub API and should not be optional.
• Add allowed_values validation: allowed_values is now required when value_type is single_select or multi_select, and rejected for string and true_false types via CustomizeDiff.
• Add checkOrganization guard: Create/Read/Update/Delete now validate that an organization is configured.
• Improve error messages: All API errors are now wrapped with fmt.Errorf for better context.
• Add resource Description.

Tests

• Added validation test: rejects allowed_values for string type.
• Added validation test: requires allowed_values for single_select type.

Testing

• go build ./... — passes
• go vet ./... — passes
• New acceptance tests added for validation logic

[github-actions]

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