Fixes NotEquals filter for null and typed values

[arotolo]

Corrects the behavior of the NotEquals filter for various scenarios:

• Handles null values correctly: Previously, Filter(NotEquals, x => x.Member, null) for scalar paths generated JSON_EXTRACT(...) <> NULL, which always evaluates to false in SQL. The filter now emits IS NOT NULL, ensuring that the "member is present" test returns the expected rows, consistent with list-member paths and Equals for nulls.
• Compares typed values consistently: Addresses issues where NotEquals with DateTime, DateTimeOffset, or numeric values could incorrectly include excluded rows.
  • DateTime and DateTimeOffset values were bound as culture-formatted text, not the serializer's ISO format.
  • Numeric members lacked the necessary CAST(… as NUMERIC) for comparison.
    These operators now share a unified type dispatch logic for both scalar and list-member paths, guaranteeing values are bound and cast correctly (e.g., DateTime uses serializer format, numeric values are cast to NUMERIC), ensuring accurate exclusions.

Introduces new unit tests to thoroughly cover NotEquals behavior for null, date-time, and numeric values on both scalar and list-member properties.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Ensure date-time comparisons use the serializer’s actual representation and add execution-based list-member coverage.

Review effort: Lite
Findings: None

What changed in this PR

Fixes NotEquals filtering for null, date-time, and numeric values across scalar and list-member paths.

Changes:

• Adds IS NOT NULL handling for null comparisons.
• Applies typed comparisons and numeric casts consistently.
• Adds regression tests and changelog documentation.

File	Review summary
TychoDB/​FilterBuilder.cs	Centralizes comparison rendering. Custom date-time formats may not match serializer output for scalar and list-member comparisons.
TychoDB.UnitTests/​NotEqualsValueTests.cs	Covers typed values, but list-member cases verify SQL rather than execution results.
TychoDB.UnitTests/​NotEqualsNullTests.cs	Covers null and presence semantics.
TychoDB.UnitTests/​NotEqualsDateTimeTests.cs	Covers date-time exclusions.
CHANGELOG.md	Documents the fixes.

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