fix(flags): multi-condition flags with static cohorts returning wrong variants

[haacked]

When a feature flag has multiple conditions and one contains a static cohort, the SDK now correctly falls back to the API instead of evaluating subsequent conditions locally and returning incorrect variants.

Introduce RequiresServerEvaluation exception to distinguish between:

• Missing server-side data (static cohorts) → immediate API fallback
• Evaluation errors (bad regex, missing properties) → try next condition

Changes:

• Add RequiresServerEvaluation exception class
• Update match_cohort() to throw RequiresServerEvaluation for static cohorts
• Update match_property_group() to propagate RequiresServerEvaluation
• Update match_feature_flag_properties() to handle both exception types
• Update client.py to catch both exceptions for API fallback
• Export RequiresServerEvaluation in __init__.py
• Add test for multi-condition static cohort scenario

Follows same pattern as Node.js SDK fix: PostHog/posthog-js#2465

[greptile-apps]

Additional Comments (1)

1. posthog/feature_flags.py, line 126-141 (link)

   logic: RequiresServerEvaluation isn't caught and propagated in the recursive flag dependency evaluation. If a dependent flag contains a static cohort, the exception will propagate but won't be cached in evaluation_cache, which could cause issues if the same dependency is checked again.

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