Move workflow_reuse.py fetch onto audit.contents_text #3106

spec/audit.py now has contents_text(content), the one decoder for a GitHub contents API response: it returns the decoded text, or None where the response holds no inline base64 content, a directory listing or a too-large file among them.

spec/workflow_reuse.py fetch (around line 73) still decodes the response itself, with its own encoding != "base64" check and base64.b64decode(...).decode("utf-8", "replace"). Move it onto audit.contents_text, as spec/fidelity_honesty.py fetch already is, so a later change to the decode is made once.

Found while driving the pull request that added the helper, which moved every such site in the files it touched.