Fix hover documentation for intersected properties

[cuishuang]

Fixes microsoft/TypeScript#63640.

Synthetic intersection properties can have multiple declarations but no value declaration. Hover documentation previously only inspected the value declaration, causing the merged JSDoc to be lost.

This change falls back to gathering and deduplicating documentation from all symbol declarations and adds a regression test.

This change was developed with assistance from OpenAI Codex. I reviewed and understand the patch and will respond to review feedback.

[Copilot]

Pull request overview

Fixes hover/QuickInfo documentation loss for synthetic intersection (and union) properties that have declarations but no value declaration by falling back to aggregating JSDoc from all symbol declarations, with a regression fourslash test.

Changes:

• Refactors documentation collection into getDocumentationFromDeclarations (gather + dedupe across declarations).
• Updates hover documentation retrieval to fall back to declaration-based docs when no info.declaration is available.
• Adds a fourslash regression test asserting merged hover documentation for an A & B property.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/ls/jsdoc.go	Extracts a helper to gather/dedupe documentation text across a provided declaration list.
internal/ls/hover.go	Adds a hover fallback path to use aggregated docs when there is no value declaration (synthetic union/intersection properties).
internal/fourslash/tests/hoverIntersectionPropertyDocumentation_test.go	Adds a regression test verifying merged hover docs for an intersection property.

[Andarist]

this issue is already being fixed by my older PR: #3663