<!-- Provide a brief summary of your changes -->
## Motivation and Context
Currently the `dns` and `http` based auth flows share code well in the
publisher tool. However there is a lot of duplication in the auth
endpoint. This on it's own is not a problem but it does make
enhancements to the proof record (the `v=MCPv1; ...` string) harder to
keep in sync.
This PR adds a `common.go` file to share logic between the two.
This supports work I am prototyping for
https://github.com/modelcontextprotocol/registry/issues/482.
## How Has This Been Tested?
Added new test cases for the permissions. Ran them before and after the
refactoring to ensure no behavior change.
Ran existing UTs.
Ran the integration tests.
Manually exercised the flows locally.
## Breaking Changes
None.
## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [ ] I have added or updated documentation as needed
## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->
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