Test the untested paths; drop two unreachable helpers

[bvdmitri]

Codecov reported 63 lines without coverage on main (96.64%). This adds tests for the reachable ones and removes two helpers nothing can reach. Locally, on Julia 1.13 with CI's variables, line coverage goes to about 98.4%.

Newly tested

• the in-place marginal API, message_passing_marginalrule!, and its refusal of a rule with no in-place form (it had no test at all);
• the result card's modes (messages and marginals under the default algorithm; marginals, and both, under another), a log scale computed from the inputs, and the HTML rows of incoming log scales;
• a node given by a function, and a rule for a node with no declaration (through message_passing_rule);
• the two-argument forms of NodeSpec, RuleCoverage, DependenciesSpec, TargetDependencies, Drawing, RuleContext and NodeFunctionLogPdf, and the coverage table's labels for parametric algorithms;
• a RuleNotFoundError for arguments that are not a RuleArgs;
• annotations with no store and a rule's own annotations; a joint of four members; RuleArgs from its parts;
• interface_groups, selection_arity, a redefined node's registry entry;
• the macros' errors for a malformed target or cluster member; undefined log scales from a fallback or a custom cause.

Removed

• input_labels: no caller.
• logscale_source(::Nothing): unreachable, since an undeclared log scale is an UndefinedLogScale, never a number.

Left untested, as defensive branches with no realistic input: a few fallbacks in diagnostics.jl (an unmatched type description, a rule input naming no interface, a method lookup that cannot fail) and held for a cluster in rulespec.jl.

No user-visible change, so no CHANGELOG entry.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.25%. Comparing base (6194f45) to head (7df6bd5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   96.64%   99.25%   +2.60%     
==========================================
  Files          25       25              
  Lines        1878     1875       -3     
==========================================
+ Hits         1815     1861      +46     
+ Misses         63       14      -49     

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