Create changelog for v1.19.0 release

[Copilot]

This PR creates the complete changelog for the v1.19.0 release by updating the version headers and documenting all significant changes merged since v1.18.2 (August 14, 2025).

Changes Made

• Updated cli/azd/CHANGELOG.md version header from "1.19.0-beta.1 (Unreleased)" to "1.19.0"
• Updated cli/version.txt from "1.19.0-beta.1" to "1.19.0"
• Added comprehensive changelog entries organized into Features Added, Bugs Fixed, and Other Changes
• Updated .vscode/cspell-github-user-aliases.txt to include Saipriya-1144 for external contributor attribution
• Removed azd agent integration feature from changelog per team feedback
• Cleaned up duplicate empty sections from the changelog structure

Key Features in v1.19.0

Major Features:

• Provisioning layers support - Allows defining multiple infrastructure layers in azure.yaml with infra.layers configuration
• Extension management improvements - Better ordering, version display, and source disambiguation for azd extension commands
• Enhanced authentication - Added claims support for device-code authentication flows
• Service principal creation improvements - Prompts for Service Tree ID when required by tenant policies

Important Fixes:

• Improved deployment log readability with better spacing
• Fixed CloudShell telemetry emission after first run
• Enhanced warning message formatting consistency

Platform Updates:

• Updated to Go 1.25.0
• Updated Aspire sample to version 9.4
• Updated extension registry with microsoft.azd.extensions v0.5.0

The changelog follows the established format with proper PR links and external contributor attribution where applicable.

Fixes #5706.

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