chore(deps): pin js-yaml 4.x to 4.3.2 to clear npm audit

[SebTardif]

Pin the 4.x js-yaml line to 4.3.2 so the required npm audit check can pass again.

Problem

npm audit --audit-level=high fails on main and on every open Dependabot PR:

• GHSA-2883-xcg3-v3hh: maxTotalMergeKeys does not count empty merge sources
• Affected: js-yaml >=4.0.0 <4.3.2
• Pulled in by mocha and @secretlint (via @vscode/vsce)

Required check npm audit therefore blocks:

• chore(deps-dev): bump ovsx from 1.1.1 to 1.2.0 #268 ovsx 1.1.1 -> 1.2.0
• chore(deps-dev): bump @types/node from 26.4.1 to 26.5.1 #269 @types/node 26.4.1 -> 26.5.1
• chore(deps-dev): bump vscode-extension-tester from 8.26.0 to 8.26.1 #270 vscode-extension-tester 8.26.0 -> 8.26.1
• chore(deps): bump the actions group with 4 updates #271 actions group (codeql-action)

Change

• Add override js-yaml@4: 4.3.2 so the 5.x copy used by vscode-extension-tester stays on 5.x
• Lockfile: js-yaml 4.3.1 -> 4.3.2

Validation

• npm audit --audit-level=high reports 0 vulnerabilities
• npm ls js-yaml shows 4.3.2 (overridden) and 5.4.1 (tester)
• npm run check after the pin