fix(hydro_lang): move KeyedStream trait constraints from impl to methods

[sim-hash]

Moves K: Eq + Hash constraints from impl block where clauses to individual method where clauses in KeyedStream.

fixes #2254

[Copilot]

Pull request overview

This PR refactors the KeyedStream implementation to move K: Eq + Hash trait constraints from impl block where clauses to individual method where clauses. This improves API flexibility by allowing methods that don't require key hashing/equality (like map, filter, inspect) to work with any key type, while still enforcing constraints on methods that need to group by key (like aggregations and joins).

• Removed K: Eq + Hash from four impl block where clauses
• Added K: Eq + Hash (or K: Clone + Eq + Hash where needed) to 16 individual method where clauses
• Methods requiring hashing/equality: scan, generator, fold_early_stop, first, various fold/reduce variants, value_counts, and filter_key_not_in

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MingweiSamuel]

I believe not all of the methods will need Eq + Hash, e.g. maybe not pub fn first(self), @shadaj ?

[shadaj]

I believe not all of the methods will need Eq + Hash, e.g. maybe not pub fn first(self), @shadaj ?

Actually first will require it as well, since it has to group by key to find the first per key. Will careful review of all the bounds though.

[shadaj]

Looks good!