fix: reject *args and **kwargs on tool signatures

[gyanu2507]

Fixes #3514

Registering a tool whose signature has *args or **kwargs currently succeeds, and list_tools advertises args/kwargs as a required scalar. There is no JSON object that can fill that field the way the function actually receives it, so the tool is permanently uncallable.

This raises InvalidSignature at registration, matching the existing rejection of a leading underscore in a parameter name. Resource templates still pass allow_variadic=True because they legitimately use **kwargs for URI variables that are only known at match time.

Tested for both *args and **kwargs tools, and that a **kwargs resource template still registers.

[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 #3514.

If a maintainer assigns you to #3514, 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.
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Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.