Add tests for custom rules that read arguments their function does not

[vchuravy]

Stacked on #3732, which is stacked on #3734 (Enzyme_jll 0.0.300).

A custom rule may read an argument that the body of its function never reads. Attribute inference on the primal body used to mark such an argument readnone, which broke the rule in two ways. Since EnzymeAD/Enzyme#3352 and #3353, which are in Enzyme_jll 0.0.300, nothing infers readnone or writeonly on the parameters of a function with a custom rule. This PR adds the regression tests from #3610, without its code changes:

• test/rules/unread_const_arg.jl (Custom forward rule argument becomes jl_nothing across a @noinline helper (SIGSEGV) #3570): a Const argument read only by the rule, behind a @noinline function. Covers forward, combined reverse and split reverse mode, and a constant callee that calls a function with a rule.
• test/rules/custom_rule_inline_roots.jl: an aggregate with inline roots whose data half is read only by the rule. The DSEPass of middle_optimize! used to delete the caller's store into it.

Run locally with Enzyme.jl main (no #3610 code changes):

	0.0.299, 1.12	0.0.299, 1.13	0.0.300, 1.12	0.0.300, 1.13
unread_const_arg.jl	segfault	segfault	9/9	9/9
custom_rule_inline_roots.jl	1 of 8 fail	4 of 8 fail	8/8	8/8

Not run locally on 1.10 or 1.11.

Fixes #3570.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (0dfa740) to head (c25c0b6).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3736       +/-   ##
===========================================
+ Coverage   49.08%   79.90%   +30.82%     
===========================================
  Files          58       71       +13     
  Lines       23379    24820     +1441     
===========================================
+ Hits        11476    19833     +8357     
+ Misses      11903     4987     -6916     

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