fix: make ClickHouseDataReader.IsClosed reflect disposal

[fallintoplace]

Summary

ClickHouseDataReader.IsClosed always returned false, even after Close() or Dispose().

• Fix: Track the closed state in Dispose(bool).
• Test: Add a regression test for Close().
• Changelog: Add a bug-fix fragment.

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG

[Copilot]

Pull request overview

This PR fixes ADO.NET reader lifecycle reporting by making ClickHouseDataReader.IsClosed accurately reflect disposal/closure, aligning behavior with expected DbDataReader semantics and preventing misleading state checks by consumers.

Changes:

• Track closed state via a new isClosed field and set it during Dispose(bool).
• Update IsClosed to return the tracked closed state.
• Add a regression test asserting IsClosed becomes true after calling Close(), plus a changelog fragment.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ClickHouse.Driver/ADO/Readers/ClickHouseDataReader.cs	Adds isClosed tracking and returns it from IsClosed, set during disposal so Close() (which calls Dispose()) is reflected.
ClickHouse.Driver.Tests/ADO/DataReaderTests.cs	Adds regression coverage to ensure IsClosed flips to true after Close().
changelog.d/fix-reader-isclosed.fixes.md	Documents the user-visible bug fix in a changelog fragment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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