Read the API token from CONVERTAPI_TOKEN when none is set in code - #96

Part of 3.3.0. When no token is set in code, the SDK now reads it from the CONVERTAPI_TOKEN environment variable, the name the README and the examples already use.

What changed

• Token lookup: a token set in code (new ConvertApi("...") or ConvertApi.ApiToken) always comes first. Otherwise each request reads CONVERTAPI_TOKEN, with surrounding whitespace trimmed. new ConvertApi() now works without a token in the source code.
• Clear error before anything is sent: with no token in code and none in the variable, uploads, ConvertAsync and GetUserAsync throw a ConvertApiException with StatusCode 401. The message says how to set the token and links to the token page, tagged utm_source=dotnet-sdk&utm_content=error-no-token. Before this change, the SDK uploaded the input files first and then got a 401 from the API. The exception type and status code stay the same, so code that handled that 401 keeps working. Response holds the same text as Message.
• new ConvertApi("") and new ConvertApi(null) still throw ArgumentNullException, and the message now names CONVERTAPI_TOKEN.
• ConvertApi.TokenEnvironmentVariable is a new public constant holding the variable's name.
• Requests that also work without a token (the OpenAPI lookup) send the token if there is one and nothing otherwise, as before.
• README: the "Set the API token" section says that from 3.3.0 on, new ConvertApi() reads the variable by itself. The snippets keep Environment.GetEnvironmentVariable("CONVERTAPI_TOKEN"), because they must also work with 3.2.2, which is what nuget.org serves today.
• CHANGELOG: entries under [Unreleased] / Added.

How it was tested

• The CI steps, run locally: restore, Release build, dotnet test (9 of 9 pass: the 4 existing tests and 5 new ones in ConvertApi.Tests/TokenTests.cs), and pack. The build has the same 3 warnings as main. NU5048 (PackageIconUrl) was already there and is handled separately in 3.3.0.
• The new unit tests use a fake IConvertApiHttpClient that records the bearer token of each request. They check that:
  • a token set in code wins over the variable;
  • with no token in code, the variable is used, trimmed, for the upload, the conversion and GetUserAsync;
  • with no token at all, a 401 ConvertApiException is thrown and no request is sent;
  • GetUserAsync with no token sends nothing;
  • new ConvertApi("") names the variable.
    The tests restore the static token, the static client and the variable afterwards.
• Against the staging API, using a console app built on this branch:
  • new ConvertApi() with only CONVERTAPI_TOKEN set converted Examples/TestFiles/test.docx to a 146,734-byte PDF. GetUserAsync worked, and DeleteFilesAsync removed both server files.
  • With no token anywhere: a 401 ConvertApiException with the message above, and no upload.
  • With a wrong token in code and a valid variable: the API answered 401, which shows the token set in code was the one sent.

🤖 Generated with Claude Code