Adding support for new COUNT aggregator for some sorted set commands - ZINTER, ZINTERSTORE, ZUNION, ZUNIONSTORE

[petyaslavova]

Add COUNT aggregator support for sorted set commands (ZINTER, ZINTERSTORE, ZUNION, ZUNIONSTORE)

This PR adds support for the new COUNT aggregation mode introduced in Redis 8.8.0 for sorted set aggregate commands: ZINTER, ZINTERSTORE, ZUNION, and ZUNIONSTORE. The COUNT aggregator ignores original element scores and instead counts weighted set membership — each element's resulting score is the sum of the weights of the input sets that contain it. When all weights are 1 (the default), the score simply equals the number of input sets containing the element.

The implementation updates the _zaggregate helper in redis/commands/core.py to accept "COUNT" as a valid aggregate option alongside the existing SUM, MIN, and MAX modes.
The error message for invalid aggregate values has been updated accordingly.
Docstrings for zinter, zinterstore, zunion, and zunionstore have been expanded to document all four aggregation modes with clear descriptions and score formulas.

Comprehensive tests have been added across all test suites — sync, async, sync cluster, and async cluster — covering both basic COUNT aggregation and COUNT with custom weights.

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}