Add XML docs for query execution extensions

[Rajesh270712]

Summary

• Add XML documentation comments for the public query execution extension methods in SqlKata.Execution/Query.Extensions.cs.
• Cover the synchronous, asynchronous, transaction, pagination, and chunking helpers in this file.
• Keep this as a focused pass toward XML Documentation on all public methods #35 rather than claiming every public API in the repository is now documented.

Linked issue / bounty

• Related to XML Documentation on all public methods #35
• Bounty proof: https://oss.issuehunt.io/r/sqlkata/querybuilder/issues/35

Problem

The public execution extension APIs currently have no XML comments, so IDE consumers do not get inline summaries, parameter descriptions, return-value descriptions, or cancellation-token guidance for these helpers.

Fix

This PR adds XML summaries, <param>, <returns>, and relevant async/cancellation notes for the public methods in Query.Extensions.cs without changing runtime code, signatures, dependencies, or generated files.

Tests

• git diff --check HEAD^ HEAD
• XML doc coverage script: public_members=62, missing_xml_docs=0
• dotnet restore
• dotnet build --no-restore passed with 0 Warning(s) and 0 Error(s)
• dotnet test --no-restore --no-build --filter 'FullyQualifiedName!~MySqlExecutionTest' passed: 334 passed, 1 skipped, 0 failed
• dotnet test --no-restore --no-build was also attempted; it reached 334 passed and 1 skipped, with 9 failures from the MySQL integration fixture because SQLKATA_MYSQL_HOST / a MySQL test service was not configured in this environment

Risk / maintainer notes

• Documentation-only diff in SqlKata.Execution/Query.Extensions.cs.
• No runtime behavior, dependency, lockfile, generated tracked file, or public signature changes.
• I am not claiming this completes XML docs for the entire repository; it is a focused first pass for the query execution extensions.