In fetch_resource_metadata_from_url, a JSON parse failure on the
response body caused a fatal AuthError::MetadataError, preventing
discover_metadata() from falling through to direct
.well-known/oauth-authorization-server discovery (Strategy B).
MCP servers that return HTTP 200 with non-JSON content (e.g. HTML)
at their base URL caused the OAuth flow to abort entirely, even
when the server had a valid .well-known/oauth-authorization-server
endpoint.
Return Ok(None) on parse failure, consistent with how HTTP errors
are already handled in the same function.
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