macros: report unbound metavariables directly

[SynapLink]

Fixes #95943.

When a macro expansion contains an unbound $ident, report it as a missing macro parameter instead of keeping the generic fragment parser error as the primary diagnostic.

This keeps the existing typo suggestions and available-metavariable notes.

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[anoshyn]

Looks good overall. One edge case I noticed: the primary diagnostic seems to be updated in the no-suggestion path, but typo / other-matcher paths may still keep expected expression, found '$' as the headline while only adding the metavariable-specific help/note.

It may be worth moving the primary message/span rewrite to immediately after detecting $ident, before branching into typo / other-matcher / available-name suggestions, so all unbound metavariable cases consistently report cannot find macro parameter ....

Also, since this PR is actively addressing #95943, consider claiming the issue with @rustbot claim.

Approve with that as a non-blocking review comment.