fix(ocm): reject extraIdentity with non-string values via schema

[ccwienk]

Summary

• YAML `safe_load` parses unquoted integers (e.g. `architecture: 386`) as `int`; downstream code calling `ComponentDescriptor.from_dict()` then crashes with a confusing `dacite.WrongTypeError`
• Root cause fix: add `additionalProperties: {type: string}` to `identityAttribute` in the OCM schema so documents with non-string `extraIdentity` values are rejected at validation time with a clear error
• The primary fix is in the upstream repository (quoting the GHA expression), this PR ensures the schema enforces the invariant for any caller

Changes

• `ocm/ocm-component-descriptor-schema.yaml`: `identityAttribute` now requires all values to be strings
• `test/ocm/componentmodel_test.py`: regression test verifies `ComponentDescriptor.validate()` rejects a descriptor with `architecture: 386` (integer)

Test plan

• `pytest test/ocm/` — all 17 tests pass

[gardener-prow]

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[ccwienk]

/hold (still wip)