docs: add release notes for v0.19.0

[Pouyanpi]

related: #1519

[github-actions]

Documentation preview

https://nvidia-nemo.github.io/Guardrails/review/pr-1525

[greptile-apps]

Greptile Overview

Greptile Summary

Added release notes for v0.19.0 documenting LangChain 1.x support and bug fixes.

• Added support for LangChain 1.x with new content blocks API for reasoning traces and tool calls
• Documented fix for TypeError in Colang 2.x chat due to incorrect State/dict conversion
• Documented fix for async streaming support in ChatNVIDIA provider patch
• Follows consistent formatting with previous release notes entries

Confidence Score: 5/5

• This PR is safe to merge with no risk
• Documentation-only change that accurately reflects the commits included in v0.19.0. The release notes follow the established format and correctly document the key features and bug fixes from the corresponding pull requests.
• No files require special attention

Important Files Changed

File Analysis

Filename	Score	Overview
docs/release-notes.md	5/5	Added v0.19.0 release notes documenting LangChain 1.x support and two bug fixes

Sequence Diagram

sequenceDiagram
    participant Dev as Developer
    participant PR as PR #1525
    participant Docs as release-notes.md
    participant Repo as Repository
    
    Dev->>PR: Create PR for v0.19.0 release notes
    PR->>Docs: Add v0.19.0 section
    Docs->>Docs: Document LangChain 1.x support
    Docs->>Docs: Document State/dict conversion fix
    Docs->>Docs: Document async streaming fix
    PR->>Repo: Merge to develop branch
    Repo->>Repo: Tag v0.19.0
    Repo->>Repo: Publish GitHub release

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[greptile-apps]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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