feat(extensions): add stddev aggregate function to percentile module

[kelvincesar]

This implementation computes the sample standard deviation, which is the same convention used by most SQL engines (e.g. PostgreSQL).

Description

• Extends the percentile extension by adding a stddev aggregation;
• Implements the stddev(expr) aggregate using Welford’s algorithm;
• Adds extension tests for stddev in both grouped and ungrouped queries.

Motivation and context

Standard deviation is a common statistical measure that can be helpful in many use cases.

This resolves #1060.

[turso-bot]

Please review @pedrocarlo

[Copilot]

Pull request overview

This PR adds a stddev aggregate function to the percentile extension, implementing sample standard deviation using Welford's online algorithm. This is consistent with the convention used by most SQL engines like PostgreSQL.

• Implements stddev(expr) aggregate function using Welford's algorithm for numerical stability
• Adds comprehensive test coverage for both grouped and ungrouped queries
• Returns NULL for datasets with fewer than 2 values (sample stddev requires at least 2 observations)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
extensions/percentile/src/lib.rs	Implements the StandardDeviation aggregate function using Welford's algorithm, correctly computing sample standard deviation
testing/cli_tests/extensions.py	Adds comprehensive test coverage for stddev including edge cases (single row, two rows) and grouped aggregates

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