Add a passing fixture and test the oldest supported VS Code #132

Why: a test that only checks the error case misses the extension reporting errors on correct code. engines.vscode claims ^1.82.0, but development only happens on the current VS Code, so the minimum is never tested.

What:

• Add client/test-fixtures/passing/, and check that an empty onDiagnostics arrives. A timeout with no diagnostics doesn't count as passing.
• Run the integration tests on both 1.82.0 and stable, reading the minimum from engines.vscode. The passing fixture runs on every branch; the 1.82.0 run only on PRs and main, to keep branch pushes fast.
• Keep @types/vscode at 1.82.x, so code can't use APIs newer than the minimum.

References:

• vscode-azureapicenter reads the minimum from package.json (semver.minVersion(pkg.engines.vscode)): https://github.com/microsoft/vscode-azureapicenter/blob/dev/.vscode-test.mjs
• vscode-coder tests both its minimum VS Code version and stable: https://github.com/coder/vscode-coder/blob/main/.vscode-test.mjs