Fix union types for @param-closure-this tags

[ace-of-aces]

Note

The code in this PR was written and reviewed by AI (GPT-6 Astra). I am not really proficient in Rust, but I tested the outcome and read through the code and tests.

Fixes union types in @param-closure-this: an annotation such as FirstContext|SecondContext previously failed to resolve the closure's $this binding. This fixes the same issue that existed in Intelephense #3512.

Closure bindings now resolve through the shared type engine and retain all class alternatives. The forward walker installs the binding before applying type guards, preserving narrowing in nested callbacks while allowing explicit rebinding. Call resolution also carries the alternatives through $this, self::, and static:: chains, with hover and navigation reflecting the union.

Adds regression tests for resolution, narrowing, diagnostics, hover, and navigation, plus a working demo with runtime assertions.

Screenshot of a working example

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.