fix(auto): don't treat strings as owned on Julia 1.12

[MilesCranmerBot]

Summary

ismutabletype(String) is true on Julia 1.12 (memory-based layout), so the owned-type tracker's generic mutable-type rule classified strings as owned. Semantically strings have value semantics with no user-facing in-place mutation API, and the package's own model treats immutable values as safe to pass around.

This caused opaque/unresolved calls to spuriously report consuming string arguments (e.g. any @safe wrapper forwarding a String through a package function whose summary cannot be resolved).

AbstractString is now classified as neither tracked nor owned.

Test plan

• New regression testsets: string forwarding through an opaque callee, and a string stashed into a Dict then reused (both previously violated, now clean).
• BORROWCHECKER_ONLY_AUTO=1 julia --project=. -e 'using Pkg; Pkg.test()': green; the two remaining @test_broken cases are fixed by fix(auto): stop false-positive generators in effect summarization #79 and intentionally still marked broken here.

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[MilesCranmerBot]

Addressed in f012a71: the exemption is now (T === String || T <: SubString) instead of T <: AbstractString, with a comment noting that user-defined mutable string subtypes must stay tracked. Verified that a hypothetical mutable struct MutStr <: AbstractString remains tracked+owned while String/SubString stay exempt.