Prepare 1.1.271-preview release notes

[Mike Krüger (mkrueger)]

Summary

• Complete the changelog coverage for merged changes since the previous release-note update, including MCP transport/subscriptions, pagination, transactional batches, query explanations, doctor diagnostics, localization, and recent fixes.
• Stamp the release as 1.1.271-preview — 2026-10-02, retaining an empty ## Unreleased section above it.
• Add a Copilot instruction requiring the Unreleased heading to remain when preparing releases.

Validation

• Verified coverage of all 32 first-parent PR merges since release notes Prepare 1.1.209-preview release notes #199; already-documented features remain in their existing release sections.
• Preserved previous release sections.
• git diff --check and git diff --cached --check passed.
• Documentation-only change; builds and tests not run.

[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation accurately reflects the merged changes and preserves the required changelog structure.

Review effort: Balanced
Findings: None

What changed in this PR

Prepares the 1.1.271-preview release documentation and codifies the changelog release convention.

Changes:

• Adds comprehensive release notes dated 2026-10-02.
• Retains an empty Unreleased section.
• Documents this requirement for future releases.

File	Description
CHANGELOG.md	Adds and stamps the release notes.
.github/​copilot-instructions.md	Requires preserving the Unreleased heading.

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[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall line coverage in commit 4684e19 in the dev/mkrueger/docs/re... branch remains at 66%, unchanged from commit dc14571 in the main branch.