[cherry-pick] Re-enable Chat Sandbox home-read smoke test by re-warming chat after restart

[vs-code-engineering]

Cherry-pick of #325594 from release/1.129.

Problem

The Chat Sandbox › allows reading a home directory file configured in allowRead smoke test is flaky. It failed 9/20 times in the flaky-test pipeline and was red on the release build, so #325575 skipped it on release/1.129 to unblock the build:

• Failing release build: https://dev.azure.com/monacotools/Monaco/_build/results?buildId=455304&view=logs&j=e352877c-ff47-5dec-32e2-b206099d9704&t=e3dd7514-b813-5e5e-156d-669f9d8e2cfc&s=a56e7651-4213-56af-9488-c25fe636c10c
• Flaky-test pipeline (9/20): https://dev.azure.com/monacotools/Monaco/_build/results?buildId=455265&view=results

The observed error:

Error: Timed out waiting for response matching /HOME_READ_ALLOWED_EXIT_CODE=(\d+)/ in
'div[id="workbench.panel.chat"] .interactive-item-container.interactive-response .rendered-markdown'
  at Chat.pollForResponseText (test/automation/out/chat.js:185:15)

This PR re-enables the test with a real fix.

Root cause

The test (and the sibling allowNetwork test) call restartWithUpdatedSandboxSettings, which restarts the app so the chat.agent.sandbox settings reload deterministically (introduced in #325532). A full restart tears down the extension host along with the warmed-up chat participant and its mock LLM connection.

The helper only waited for the chat view DOM via waitForChatView() before sending the probe message. waitForChatView waits for the chat view to render, not for chat to be ready to answer. On loaded CI agents the first probe races an unready chat, the mock response never renders, and pollForResponseText times out after 120s.

#325532 fixed one race (settings not reloading → HOME_READ_ALLOWED_EXIT_CODE=1) but introduced this one (chat not re-warmed after restart → probe timeout).

Fix

• Re-run the existing warmUpChat retry loop (already used on cold start, retries up to 180s) inside restartWithUpdatedSandboxSettings, right after waitForChatView(), so the probe is only sent once chat can reliably reach the mock LLM server.
• Remove the it.skip added in test: skip flaky chat sandbox home-read smoke test #325575 to re-enable the test.

Verification (local A/B)

Ran the same loop locally against the installed Insiders build (--build/--stable-build pointed at Visual Studio Code - Insiders.app, 1.129.0-insider):

	HOME_READ probe timeout (the bug)	Passed	Unrelated env failures*
Without fix (post-restart warm-up disabled)	3/8 ❌	3/8	2/8
With fix	0/13 ✅	12/13	1/13

Without the fix the exact CI error reproduces (Timed out waiting for response matching /HOME_READ_ALLOWED_EXIT_CODE=(\d+)/) at ~38%, close to the CI flaky rate of 9/20 (45%). With the fix it never occurs across 13 runs (8 on a main-based branch, 5 on this release/1.129-based branch).

* Unrelated failures are Timeout of 300000ms exceeded … main.js — the outer setup/before hook timing out from the loop launching the app repeatedly; not the chat race, and present in both columns.

Targeting

This PR targets release/1.129 (where the test is skipped) so the fix lands on the release branch and flows forward into main. Closes microsoft/vscode-engineering#3280.

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