Run Windows and macOS tests weekly

[rcosta358]

Description

Closes #134.

Add weekly and manual Windows/macOS runs for client and server unit tests plus VS Code stable integration tests, with logs uploaded on failure.

Related Issues

Stacked on #146, whose dependency chain (#145, #144, #143, #138, #137) leads to main. This branch also incorporates #139 (publish the checked VSIX), #141 (server unit tests), and #142 (client unit tests), which must land before this PR. The platform schedule becomes active when the stack reaches the default branch.

Validation: 32 client unit tests, 24 server unit tests, TypeScript checks, lint, both VS Code stable integration fixture suites, and local extension packaging/installation passed on the updated branch. Earlier Windows/macOS execution (before these dependency merges): https://github.com/liquid-java/vscode-liquidjava/actions/runs/37034618182

🤖 Generated with Codex

[CatarinaGamboa]

Base branch: this PR targets codex/phase-4-test-base, which has no PR to main and doesn't contain #139 (git merge-base --is-ancestor on #139's branch and phase-4: not an ancestor). Merging here won't get the weekly workflow onto main, and the schedule only runs from the default branch. Once the stack lands, this needs its base changed to main (or to the last PR in the stack), and it should be rebased onto a base that includes #139.

Reviewed with Claude Code (reviewer + adversarial agents per PR, findings checked against the code before posting).

[rcosta358]

Retargeted this PR onto #146, which leads through the stack to main, and merged #139 into the branch (939e85e). The description now lists the additional unit-test dependencies and notes that the schedule activates once it reaches the default branch. Client/server tests, stable integration, and full extension installation passed.