fix(auth): exclude client_id from token body when using client_secret…

[excelrati]

…_basic

RFC 6749 Section 2.3 states that when using HTTP Basic authentication, the client credentials (client_id and client_secret) must be sent in the Authorization header only and must not appear in the request body.

Previously, prepare_token_auth() stripped client_secret from the body but left client_id in when client_secret_basic was in use, causing servers like Notion's MCP implementation to reject token requests due to duplicate credentials.

Fix: extend the body filter from
k != 'client_secret'
to
k not in ('client_secret', 'client_id')

Tests: update existing basic-auth assertions to reflect correct behaviour and add a dedicated regression test
(test_basic_auth_does_not_send_client_id_in_body).

Fixes #3138

Fixes #

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I am assigned to the linked issue (or it is labeled help wanted, or I'm a maintainer)
• I have disclosed any AI assistance and can explain the change in my own words
• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3138.

If a maintainer assigns you to #3138, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.