feat(laravel): support custom Eloquent builders and improve LSP responsiveness

[MingJen]

Summary

This PR introduces comprehensive support for Laravel custom Eloquent builders (#[UseEloquentBuilder], HasBuilder trait) and significantly enhances LSP
responsiveness through background processing and intelligent completion caching. Editor users will now see accurate type information and completion for
forwarded builder methods, even when using complex inheritance patterns in their models.

Changes

• Adds Laravel custom Eloquent builder support: Implements resolution for methods defined in custom builders specified via the #[UseEloquentBuilder]
  attribute or HasBuilder trait.
• Improves LSP responsiveness: Offloads file parsing (update_ast) and diagnostic publishing to background tasks, preventing the main thread from blocking
  during heavy typing.
• Implements member completion caching: Caches unfiltered member lists per-target to drastically reduce latency for subsequent completions while the user is
  typing a member name.
• Adds Laravel background warming: Automatically indexes common Laravel builder classes in the background during startup to eliminate "first-access" latency
  in Eloquent projects.
• Refines Find References logic: Scopes the hierarchy "walk-down" to only those classes that actually define the member, improving both the performance and
  accuracy of project-wide reference searches.

How It Works (Laravel Custom Builders)

1. Extraction: During the parsing phase, src/symbol_map/extraction.rs identifies #[UseEloquentBuilder] attributes and stores the builder's FQN in the
   ClassInfo.
2. Virtual Member Generation: When a Model's members are requested, virtual_members/laravel/builder.rs walks the inheritance chain to find the applicable
   custom builder and injects its instance methods as static "forwarded" methods on the Model.
3. Bridge Resolution: The reference engine (src/references/mod.rs) implements a "bridge" in the hierarchy resolution. If a search is initiated for a Model
   method, the engine automatically includes the associated Builder in its search scope (and vice versa), ensuring that calls like User::active() resolve
   correctly to UserBuilder::active().

Why Core Library Was Modified

• src/references/mod.rs & src/definition/member/mod.rs: Modified to implement the Model-Builder hierarchy bridge. This was necessary because the
  relationship between a Laravel Model and its Builder is a framework-specific inheritance pattern that isn't captured by standard PHP class hierarchies.
• src/server.rs: Background task orchestration was added to the core LSP loop to ensure that state updates (AST, indices) remain consistent while processing
  concurrent interactive requests.

Tests

• tests/integration/laravel_custom_builder.rs: Covers go-to-definition, completion, and type resolution for custom builders, including inherited builder
  attributes and model-generic substitution.
• tests/integration/laravel_references.rs: Asserts that "Find References" correctly bridges between declarations in a custom builder and usages on the
  associated Model.
• benches/custom_builder.rs: Benchmarks the overhead of custom builder resolution in large Model hierarchies.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.74194% with 76 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.80%. Comparing base (6c35273) to head (24045c8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/completion/handler.rs	79.16%	35 Missing ⚠️
src/parser/classes.rs	84.00%	8 Missing ⚠️
src/completion/builder.rs	80.55%	7 Missing ⚠️
src/references/mod.rs	94.21%	7 Missing ⚠️
src/virtual_members/laravel/builder.rs	93.91%	7 Missing ⚠️
src/virtual_members/mod.rs	88.37%	5 Missing ⚠️
src/completion/call_resolution.rs	72.72%	3 Missing ⚠️
src/lib.rs	75.00%	3 Missing ⚠️
src/definition/member/mod.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   86.71%   86.80%   +0.09%     
==========================================
  Files         170      170              
  Lines      108396   108888     +492     
==========================================
+ Hits        93996    94524     +528     
+ Misses      14400    14364      -36     

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