Guard cnv_current_version against missing csv spec.version (#4003)

##### Short description:
spec.version is not a required field in the ClusterServiceVersion
schema. If absent, the fixture silently returns None causing unexpected
behavior in multiple downstream tests.
##### More details:

##### What this PR does / why we need it:
identified during verifying
#3984
that we always assume csv has .spec.version , although it exist but
since it's optional if it's not there , We should fail early and at
right place

##### Which issue(s) this PR fixes:

##### Special notes for reviewer:

##### jira-ticket:
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Enhanced validation to ensure CSV specification version information is
properly defined, raising an error if the required version field is
missing rather than silently accepting empty values.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Geetika Kapoor <gkapoor@redhat.com>

Author: geetikakay

tests/conftest.py

@@ -956,7 +956,10 @@ def cnv_current_version(installing_cnv, csv_scope_session):
     if installing_cnv:
         return CNV_NOT_INSTALLED
     if csv_scope_session:
-        return csv_scope_session.instance.spec.version
+        version = csv_scope_session.instance.spec.version
+        if not version:
+            raise ValueError("CSV spec.version is missing (field is optional in schema).")
+        return version
 
 
 @pytest.fixture(scope="session")