test: add unit tests for BaseEnum membership support

[eeshsaxena]

Description

weaviate/str_enum.py provides BaseEnum/MetaEnum, which add in
support to enums (e.g. "ALL" in ConsistencyLevel), but had no dedicated
test. This adds test/test_str_enum.py covering:

• a member name string being contained
• an unknown string not being contained
• a member value string not counting as a membership match
• an enum member being contained
• a non-string (which lacks .name) not being contained

No source changes.

Verification

pytest test/test_str_enum.py - 5 passed. ruff check / ruff format --check clean.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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