Fix trait-bound method lookup ambiguity (#7397)

This PR improves the compiler's method-resolution step so trait bounds
are actually honoured when we call `T::method()`.

Before this change we kept every trait method in scope and emitted a
"multiple applicable items" error even when the generic bound singled
out one trait. Now we capture which trait each candidate comes from,
derive the active trait obligations, and filter the list before picking
a candidate. Ambiguity is still reported for real conflicts, but
constrained calls now resolve cleanly instead of failing.

Also improves the ambiguous trait hint diagnostic to prefer generic type
names when emitting the "Multiple applicable items in scope" diagnostic.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

Author: tritao

sway-core/src/semantic_analysis/namespace/mod.rs

@@ -22,7 +22,9 @@ pub(crate) use trait_map::IsImplInterfaceSurface;
 pub(crate) use trait_map::IsImplSelf;
 pub(super) use trait_map::ResolvedTraitImplItem;
 pub(crate) use trait_map::TraitEntry;
+pub(crate) use trait_map::TraitKey;
 pub use trait_map::TraitMap;
+pub(crate) use trait_map::TraitSuffix;
 pub use trait_map::TryInsertingTraitImplOnFailure;
 
 use sway_types::Ident;