Improve documentation quality in generated JSON Schema

## Description

Extracted from #1609 (section "3. Documentation Quality"). The generated `citrus-testcase.json` schema has several documentation quality issues that affect IDE autocompletion hints, developer experience, and schema readability.

## Tasks

- [ ] **3.1 All 28 definitions lack top-level descriptions**
None of the entries in `definitions` have a `description` at the definition level.

- [ ] **3.2 Inconsistent trailing periods (120 of 1814 descriptions lack trailing period)**

- [ ] **3.3 `Element` definition properties (Selenium) have titles but no descriptions**

- [ ] **3.4 `selenium.javaScript.argument` has no `type` constraint**

- [ ] **3.5 Opaque definition names: `Expression-1`, `Expression-2`**
Different expression types distinguished only by a numeric suffix. Better: `PathValueExpression` / `PathExpression`.

- [ ] **3.6 Redundant title/description on array items**
Array items repeat parent metadata verbatim.

## Related

Parent epic: #1609

Start with the generated citrus-testcase.json schema and inspect its definitions, Element properties, selenium.javaScript.argument, and Expression-1/Expression-2 entries. Read parent issue #1609 for the broader context, then address the six listed documentation and schema-quality tasks; the work is done when those inconsistencies and omissions are resolved throughout the generated schema.

Written by the indexing model from the issue text.