fix(rails): exclude MimeNegotiation::InvalidType by default

[moladukes]

Closes #159.

MimeNegotiation::InvalidType is raised when a client sends a malformed Accept / Content-Type header, almost always scanner traffic. Rails maps it to a 406 via ActionDispatch::ExceptionWrapper.rescue_responses, so it's not an app bug. Same category as the already-excluded BadRequest / UnknownFormat / ParseError.

• Adds the class to default_excluded_exceptions
• Updates the generator template's docs
• Adds spec/posthog/rails/configuration_spec.rb

bundle exec rspec and bundle exec rubocop pass locally.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
spec/posthog/rails/configuration_spec.rb:15-27
The team's coding standard prefers parameterised tests. The two "excludes … by default" examples are identical in structure and are ideal candidates for a shared iteration. A loop-based approach also makes it trivial to extend coverage to every class in `default_excluded_exceptions` without repeating the expectation.

```suggestion
    {
      'ActionController::RoutingError' => ActionController::RoutingError.new('No route matches'),
      'ActionDispatch::Http::MimeNegotiation::InvalidType' =>
        ActionDispatch::Http::MimeNegotiation::InvalidType.new('"foo" is not a valid MIME type')
    }.each do |name, exception|
      it "excludes #{name} by default" do
        expect(config.should_capture_exception?(exception)).to be false
      end
    end
```

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