Migrate three assists to SyntaxEditor editing

[Sangria-hbm]

Summary

This PR migrates three ide-assists handlers away from mutable-tree editing patterns (ted::*, builder.make_mut, builder.make_syntax_mut) toward SyntaxEditor/SyntaxFactory-based edits.

Affected handlers:

• extract_struct_from_enum_variant
• extract_type_alias
• generate_fn_type_alias

Motivation

This is part of the ongoing migration tracked in #18285 (and related architecture goal in #15710): reducing mutable syntax editing usage so we can continue moving toward cleaner immutable editing internals.

What Changed

extract_struct_from_enum_variant

• Reworked edit flow to use SyntaxEditor and SyntaxFactory mappings.
• Removed mutable-tree paths (ted::*, builder.make_mut, builder.make_syntax_mut) from this handler.
• Migrated import insertion to insert_use_with_editor.
• Preserved behavior around comment/attribute carry-over and visibility propagation.
• Added regression test: extract_fix_references_with_turbofish.

extract_type_alias

• Switched alias/type replacement construction to SyntaxFactory::with_mappings + edit.add_mappings(...).
• Fixed duplicated generic-parameter emission when a selected type repeats the same generic (e.g. (T, T)).
• Added regression test: dedup_repeated_generic_params.

generate_fn_type_alias

• Migrated alias construction/insertion to SyntaxFactory + mapped SyntaxEditor edits.
• Fixed impl-generic + self rendering for unnamed-params alias generation.
• Added regression test: generate_fn_alias_unnamed_impl_generics_on_self.

Testing

Executed in the rust-analyzer workspace:

cargo test -p ide-assists -- handlers::extract_type_alias::tests handlers::generate_fn_type_alias::tests handlers::extract_struct_from_enum_variant::tests
cargo fmt -- --check

Result:

• 68 passed; 0 failed

Notes

• This PR is intentionally scoped to three handlers to keep review surface manageable.
• No new assist was added; this is a migration + correctness fix set.

AI Disclosure

AI tooling was used during drafting/iteration, and all code/tests in this PR were manually reviewed and verified locally.

[Sangria-hbm]

All CI checks on this PR are now passing. When you have time, could you please take a look and review? Thanks!

[Shourya742]

Use SyntaxFactory instead of make. If a required method isn’t available, add it to SyntaxFactory.

Suggested change

	self, HasGenericParams, HasName, edit::IndentLevel, make, syntax_factory::SyntaxFactory,
	self, HasGenericParams, HasName, edit::IndentLevel, syntax_factory::SyntaxFactory,

[Shourya742]

Remove make

Suggested change

	self, AstNode, HasGenericArgs, HasGenericParams, HasName, edit::IndentLevel, make,
	self, AstNode, HasGenericArgs, HasGenericParams, HasName, edit::IndentLevel,

[Shourya742]

Kindly don't change unrelated stuff

[Sangria-hbm]

Thanks for the review — I pushed follow-up commit a1a61172ae to address your comments.

Changes made:

• removed make imports in extract_type_alias and generate_fn_type_alias
• switched the whitespace insertions back to factory.whitespace(...) in both files
• reverted the unrelated comment wording change in extract_struct_from_enum_variant
• replaced remaining constructor calls in generate_fn_type_alias with SyntaxFactory methods

Could you please take another look when convenient? Thank you!