Parse JSON output from azd CLI for cleaner error messages

[Copilot]

Description

azd auth token writes JSON to stderr on failure: {"type":"consoleMessage","data":{"message":"ERROR: ..."}}. Currently this JSON appears verbatim in exceptions, producing noisy output.

Changes

• Added JSON parsing (ProcessCliErrorForDisplay): Extracts .data.message from azd JSON output, trims whitespace, falls back to raw text on parse failure or empty content
• Removed special-case substitution: Previously checked for "azd auth login" in errors and substituted with a custom constant. Now uses azd's messages directly since they're already user-friendly
• Test coverage: 7 new tests for JSON parsing (valid JSON, malformed JSON, missing fields, whitespace handling, empty messages, fallback scenarios)

Before/After

Before:

AzureDeveloperCliCredential authentication failed: {"type":"consoleMessage","timestamp":"2024-01-01T00:00:00Z","data":{"message":"\nERROR: fetching token: interactive login required"}}

After:

AzureDeveloperCliCredential authentication failed: ERROR: fetching token: interactive login required

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve AzureDeveloperCliCredential error messages by parsing azd output</issue_title>
<issue_description>When azd auth token fails, it writes JSON to stderr:

{"type":"consoleMessage","timestamp":"...","data":{"message":"\nERROR: fetching token: ..."}}

Today AzureDeveloperCliCredential includes this raw JSON verbatim in error messages, producing noisy output like:

AzureDeveloperCliCredential authentication failed: {"type":"consoleMessage","timestamp":"...","data":{"message":"\nERROR: fetching token: ..."}}

We should instead parse this JSON's .data.message field to produce cleaner errors like:

AzureDeveloperCliCredential authentication failed: ERROR: fetching token: ...

Note .data.message may contain leading/trailing whitespace that should be trimmed.

This also lets us remove special-case error handling (e.g., substituting another message when azd's contains "azd auth login") because the parsed messages from azd are already user-friendly.

When JSON parsing fails or .data.message isn't set or has an empty value, include the raw text as before.</issue_description>

<agent_instructions>This issue is for the Azure.Identity library, which you can find under sdk/identity/</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve AzureDeveloperCliCredential error messages by parsing azd output #55546

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