Add more context to dyn-compatibility diagnostic for receiver-less associated functions

[xyz-harshal]

Fixes #159492

Expands the two suggestion messages for the "no self parameter"
dyn-compatibility violation to explain why each fix works, per the issue.

Before:
help: consider turning create into a method by giving it a &self argument
help: alternatively, consider constraining create so it does not apply to trait objects

After:
help: consider turning create into a method by giving it a &self argument, so that it is accessible through the trait object's vtable
help: alternatively, consider constraining create so it is explicitly marked as not applying to trait objects

Only the two strings in DynCompatibilityViolationSolution::add_to
(rustc_middle/src/traits/mod.rs) changed — no logic, applicability, or span changes.

Added a regression test using the issue's exact example
(tests/ui/dyn-compatibility/static-constructor-prevents-dyn-no-api-guidance.rs).

Verified against a locally-built rustc; full tests/ui suite passes
(21643 passed, 0 failed). Note: next-solver compare-mode has pre-existing,
unrelated failures on 3 tests in this directory, confirmed present on main
without this diff.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @khyperia (or someone else) some time within the next two weeks.

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[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
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  • 38e36f6

[josetorrs]

Was Copilot manually invoked?

[xyz-harshal]

Was Copilot manually invoked?

Nope.

[khyperia]

thank you for the PR!

please follow the advice given by rustbot about the commit message, thanks ❤️

out of curiosity, did you write entirety of the PR description yourself, or was an LLM used in any part of the writing process?

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[apiraino]

hey @xyz-harshal

as per our LLM policy, please disable Copilot in your GitHub settings and don't trigger unprompted reviews from bots in our repository.

Thanks