fix(browser): Ensure source is set correctly when updating span name in-place in beforeStartSpan

[Lms24]

When using beforeStartSpan in browserTracingIntgration, we document that users should return a new options object rather than mutating the passed in object in-place. However, nothing keeps users from setting the name in-place like so:'

browserTracingIntegration({
  beforeStartSpan: opts => {
    opts.name = 'changed';
    return opts;
  },
}),

In such a case, we previously didn't catch the mutation and wouldn't adjust the span source attribute to 'custom' accordingly. This PR ensures we take care of this case, too. (found while writing a test for #17500)

[Lms24]

this test previously didn't properly test the source adjustment because source by default is already custom.