Closes #903 Retire DefaultAzureCredential phase 1 (awareness)

[shirasassoon]

This pull request updates the authentication documentation for fabric-cicd to deprecate the use of DefaultAzureCredential and provide clear migration guidance to explicit credential methods. It introduces a deprecation warning, reorganizes and clarifies authentication examples, and adds guidance for using explicit credential flows including Azure CLI, Azure PowerShell, and Managed Identity. The changes emphasize security best practices and provide updated code samples for different environments.

Deprecation and migration guidance:

• Added a deprecation warning for the use of DefaultAzureCredential, advising users to migrate to explicit credential methods and providing migration guidance in the documentation. (.changes/unreleased/optimization-20260325-161506.yaml)
• Updated the authentication documentation to highlight the deprecation and removal of the default credential flow, and clarified that explicit token_credential parameters should be used in all scenarios except Fabric Notebook runtime. (docs/example/authentication.md)

Expanded and improved credential examples:

• Replaced the default credential examples with explicit code samples for Azure CLI (AzureCliCredential), Azure PowerShell (AzurePowerShellCredential), and Managed Identity (ManagedIdentityCredential) authentication flows, including updated guidance and environment-specific instructions for local, Azure DevOps, and GitHub Actions scenarios. (docs/example/authentication.md) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Security and best practices:

• Added notes advising against hardcoding credentials and recommended using environment variables or secret management services, with specific guidance for SPN + Secret authentication via CLI or PowerShell. (docs/example/authentication.md)

These changes ensure users follow security best practices and have clear, up-to-date examples for authenticating with fabric-cicd in a variety of environments.

[Copilot]

Pull request overview

This PR begins deprecating DefaultAzureCredential as the implicit authentication mechanism by adding a runtime warning and updating docs/examples to prefer explicit token_credential providers across environments.

Changes:

• Added a deprecation warning when FabricWorkspace falls back to DefaultAzureCredential.
• Reworked authentication documentation and troubleshooting guidance with explicit credential examples (CLI, PowerShell, managed identity, client secret).
• Updated devtools scripts and added a changelog entry reflecting the deprecation guidance.

Reviewed changes

Copilot reviewed 7 out of 11 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/fabric_cicd/fabric_workspace.py	Emits a warning when code falls back to DefaultAzureCredential.
docs/how_to/troubleshooting.md	Updates troubleshooting guidance and marks explicit credentials as required in tables/examples.
docs/how_to/getting_started.md	Adds deprecation notice and a basic explicit-credential example.
docs/example/release_pipeline.md	Updates pipeline narrative to explicit credential flows and fixes an ADO variable reference.
docs/example/deployment_variable.md	Updates variable-injection examples to use explicit credentials and adds deprecation notice.
docs/example/authentication.md	Major rewrite of auth examples by credential type and environment, adds Fabric Notebook section.
devtools/debug_trace_deployment.py	Uses AzureCliCredential and passes token_credential explicitly.
devtools/debug_local.py	Adds explicit credential options and always passes token_credential.
devtools/debug_local config.py	Adds explicit credential options and passes token_credential.
devtools/debug_api.py	Removes DefaultAzureCredential fallback and uses validated explicit credentials.
.changes/unreleased/optimization-20260325-161506.yaml	Adds a changelog entry for the deprecation warning/migration guidance.