Improve AzureDeveloperCliCredential error messages by parsing azd output

[chlowell]

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.